732 Lynn and Brown 1955), and individuals that spend most time on the open arms of the EPM are assumed to exhibit the lowest anxiety-like response (Walf & Frye, 2007). Treatment of rodents with anxiolytic agents has been used to validate these behavioral tests, with treated animals emerging from a box or canopy sooner, and spending more time in the center of the OF and on the open arms of the EPM, than control animals (Hogg, 1996; Prut & Belzung, 2003; Yang, Gorman, & Dunn, 1999). Relatively few studies have compared the behavior of adolescent and adult rodents on these traditional behav- ioral tests of anxiety-like responses, and such studies have produced conﬂicting results. In the OF, adolescent rodents have been reported to exhibit either higher levels (e.g., Arakawa, 2005; Bronstein, 1972; Philpot & Wecker, 2008; Stansﬁeld & Kirstein, 2006) or lower levels (e.g., Candland & Campbell, 1962) of locomotor exploration than adults, and, in the EPM, adolescents have been reported to spend either more time (e.g., Doremus- Fitzwater, Varlinskaya, & Spear, 2009a; Macri, Adriani, Chiarotti, & Laviola, 2002) or less time (e.g., Doremus, Brunell, Varlinskaya, & Spear, 2003) on the open arms than adults. Other studies have failed to ﬁnd age differences in performance on these pieces of apparatus (e.g., Arakawa, 2005; Hefner & Holmes, 2007). In general, age differences in anxiety-like behavior have not been the main focus of these studies, methodological differences between studies are likely to have contributed to the conﬂicting results, and most studies have used only one behavioral test (Doremus, Varlinskaya, & Spear, 2004; Doremus-Fitzwater et al., 2009a; Slawecki, 2005). Measuring the behavioral response of adolescent and adult rats to multiple tests provides an opportunity to investigate in detail the relationship between age and anxiety-like behavior, and to examine whether these tests elicit similar or different anxiety-like responses from each other. In this study, we examined whether anxiety-like behavior in rats increases, declines, or remains stable during the transition from adolescence to adulthood, as measured in the ET, OF, and EPM. We also tested whether male and female rats behave differently on these behavioral tasks. Adult female rodents have been reported to locomote more on the OF, spend more time on the open arms of the EPM, and emerge sooner in the ET than adult males (e.g., Johnston & File, 1991; Roman & Arborelius, 2009; Swanson, 1966). Sex differences in performance on these tasks could relate to sex differences in exploratory motivation or in spatial learning ability (Lynn & Brown, 2009). However, only a small number of studies have examined sex differences on these tests during adolescence. Some studies reported that adolescent female rodents locomote more than same-aged males in an OF (e.g., Beatty & Fessler, 1976; Stewart, Skvarenina, Developmental Psychobiology & Pottier, 1975), while others failed to ﬁnd a sex difference in locomotion at this age (e.g., Masur, Schutz, & Boerngen, 1980; Slob, Huizer, & van der Werff ten Boesch, 1986). In the EPM, adolescent female rats have been reported to exhibit more open arm activity than males of the same age (e.g., Elliott, Faraday, Phillips, & Grunberg, 2004; Leussis & Anderson, 2008; Lynn & Brown, 2009), although other studies reported that this sex difference does not emerge until early adulthood (Estanislau & Morato, 2006; Imhof, Coelho, Schmitt, Morato, & Carobrez, 1993). The studies discussed so far have generally only focused on two age points, thereby precluding a detailed analysis of the ontogeny of behavioral responses to tests of anxiety-like behavior. In our study, the behavioral responses of male and female rats were examined at four ages: adolescence (postnatal days (pnd) 34 – 39), late adolescence (pnd 51 – 55), young adulthood (pnd 65 – 69), and older adulthood (pnd 104 – 109). The adolescent period in rats has been deﬁned as starting around pnd 28 (Spear, 2000a), coinciding with rising circulating gonadal hormone levels in both sexes (Gabriel, Roncancio, & Ruiz, 1992). Around pnd 35– 40, females exhibit vaginal opening and irregular ovarian cycling, while testosterone levels rise gradually in males (Gabriel et al., 1992). During late adolescence (pnd 46– 59; Tirelli, Laviola, & Adriani, 2003), females exhibit regular cycles, and males are capable of producing fertile sperm (Gabriel et al., 1992; Tentler et al., 1997). By pnd 60, rats are usually considered to be fully sexually mature. However, the male testes continue to develop into young adulthood (Knorr, Vanha-Perttula, & Lipsett, 1970), and testosterone levels peak around pnd 70 before falling to adult levels (Zanato, Martins, Anselmo-Franci, Petenusci, & Lamano- Carvalho, 1994). The four age points chosen for our study therefore represent the peak of adolescence through to full sexual maturity for both sexes. In combination with our previous research on anxiety-like behavior from early-to-late adolescence (Lynn & Brown, 2009), the results will provide detailed information about the ontogeny of anxiety-like behavior in rats from adolescence to adulthood. During the adolescent period, the hypothalamic – pituitary– adrenal (HPA) axis undergoes maturational changes in rodents. For instance, juvenile rats exhibit a hyperresponsiveness of the HPA axis and a prolonged production of corticosterone following stress compared to adults (Romeo, 2005), and adolescence is likely to be a period during which the developing stress axis is highly sensitive to perturbations (Andersen, 2003). Given that developmental changes in both gonadal and adrenal hormone production have been hypothesizd to underlie the increased susceptibility of adolescents to anxiety disorders in human beings (Sanborn & Hayward, 2003), a