736 Lynn and Brown lesser total distance compared to adults, whether meas- ured by line crossings or by tracking TDM. These age differences were maintained when body weight was included as a covariate in the analyses. The opposite result of Philpot and Wecker (2008) suggests that adolescent novelty induced locomotion is enhanced when the apparatus is scaled to size. However, it is still unclear whether scaling relative to crown-rump length results in two pieces of apparatus that are equivalent from the perspective of the subjects. The use of multiple behavioral tests, where appropriate, can avoid over-reliance on a single piece of apparatus that can vary greatly in design (Crabbe & Morris, 2004). Consistent with our OF data, the amount of time spent on the open arms of the EPM increased across age groups. A previous study has presented a factor analysis of adolescent and adult EPM performance, which suggested that similar components underlie the behavioral response of these two age groups (Doremus, Varlinskaya, & Spear, 2006). In line with our results, Doremus et al. (2003) reported that adolescent rats spend less time on the open arms than adults using apparatus that was scaled to the relative sizes of the subjects. Therefore, the age difference in EPM performance appears robust to changes in relative apparatus size. Our results also indicate that anxiety-like responses are similar in all three pieces of apparatus; for example, time spent on the open arms of the EPM correlated positively with OF locomotor activity and negatively with the number of box re-entries in the ET. The effects of age were similar across all three tests, which strengthens our conclusion that anxiety-like behavior declines from adolescence to adulthood in rats. The lack of age changes in the percentage of time in the center of the OF, and latency to exit the start box in the ET, are likely to be due to ﬂoor effects in the data, with all animals exhibiting very low numbers of entries into the center of the OF and short latencies to exit the ET start box. In this study, we compared the performance of males and females across the age groups. Our results indicated that females are quicker to leave the start box than males in the ET and locomote more than males in the OF when measured by line crossings. These ﬁndings are in line with sex differences in adult performance on these tests (e.g., Johnston & File, 1991; Roman & Arborelius, 2009; Swanson, 1966), and the OF results support previous studies of adolescents (e.g., Beatty & Fessler, 1976; Lynn & Brown, 2009; Stewart et al., 1975). The lack of age sex interactions in our data sets precludes us from drawing any conclusions about the age of emergence of sex differences in performance on these tasks. Previous researchers have concluded that male rodents are more anxious, or more fearful, than females based on response to novel apparatus such as the OF and EPM (e.g., Aguilar Developmental Psychobiology et al., 2003; Zimmerberg & Farley, 1993). However, alternative explanations for the behavioral sex differences also exist, including that females exhibit a higher motivation than males to investigate and/or escape from novel environments (Brown & Nemes, 2008). Distin- guishing between these alternative explanations will be difﬁcult based on behavioral measures alone (Hughes, 1997). In contrast to a previous study in our laboratory (Lynn & Brown, 2009), no sex differences in EPM performance were found in the current study. The previous study compared the performance of male and female adolescent rats on both the OF and EPM with similar numbers of subjects as in the current study. However, a key difference could have been the time between tests. In the earlier study (Lynn & Brown, 2009), each test was conducted two to three days apart, whereas the current study exposed the subjects to the three tests over three consecutive days. Previous research has indicated that EPM performance is particularly sensitive to prior testing in other apparatus (Carobrez & Bertoglio, 2005) and that pretest manipu- lation reduces the extent of the sex differences in EPM performance (Doremus-Fitzwater et al., 2009a). Further experiments are currently underway in our laboratory to examine the effects of repeated behavioral testing on the response of adolescent and adult rats to novel environ- ments. The main conclusion to be drawn from this study is that anxiety-like behavior on traditional behavioral tests gradually decreases from adolescence into adulthood in rats. Therefore, adolescent rodents could provide relevant information about the mechanisms by which susceptibility to anxiety disorders changes from adoles- cence to adulthood in human beings. The gradual changes in performance of rats on the ET, OF, and EPM correspond to changes in adrenal hormone production over these ages. For instance, the HPA axis undergoes developmental changes from hyperresponsiveness in the prepubertal animals to lowered responsiveness in adulthood (Romeo, 2005). A growing number of studies have shown that exposure to stress has a more pronounced and long-lasting effect on adolescent than adult rodents (e.g., Doremus-Fitzwater, Varlinskaya, & Spear, 2009b; McCormick, Smith, & Mathews, 2008; Stone & Quarter- main, 1997; for reviews, see McCormick & Mathews, 2007; Romeo, 2010). In addition, human adolescence is associated with heightened basal and stress-induced activity of the HPA axis (Lupien, McEwen, Gunnar, & Heim, 2009). Therefore, adolescence is likely to be a period during which the developing stress axis is potentially highly sensitive to perturbations, and exposure to stress during this period of life could potentially have life-long consequences for mental health in human beings.