Developmental Psychobiology (iv) Percentage of time in the center: There was no main effect of age (w23 ¼ 6.44, p ¼ .092) or of sex (w21 ¼ .75, p ¼ .385) on the percentage of time spent in the center of the OF. Elevated Plus-Maze (i) Percentage of time on open arms: There was a signiﬁcant main effect of age on the percentage of time on the open arms (F3,23 ¼ 6.56, p ¼ .002; Fig. 3). Trend analyses revealed that the percentage of open arm time increased linearly with age (p ¼ .001). There was no main effect of sex (F1,23 ¼ 2.25, p ¼ .147). (ii) Percentage of open arm entries: There was no main effect of sex (F1,23 ¼ 1.40, p ¼ .088), but there was a signiﬁcant main effect of age (F3,23 ¼ 3.09, p ¼ .047), with the percentage of open arm entries linearly increasing with age (p ¼ .007; adolescents ¼ 21.6 1.2, late adolescents ¼ 21.6 1.3, young adults ¼ 24.4 1.2, older adults ¼ 25.7 1.3). Correlations Between Performance on the ET, OF, and EPM The percentage of time on the open arms of the EPM was positively correlated with all locomotory measures on the OF: total locomotion (r ¼ .292, p ¼ .016), TDM (r ¼ .350, p ¼ .003), and the percentage of time spent mobile (r ¼ .307, p ¼ .011). The percentage of entries onto the open arms also tended to positively correlate with total locomotion (r ¼ .221, p ¼ .069) and TDM (r ¼ .222, p ¼ .069) on the OF. The percentage of time spent on the FIGURE 3 Percentage of time spent on open arms collapsed across subjects’ sex (means SEMs). A signiﬁcant main effect of age (p < .01). Ontogeny of Anxiety-Like Behavior in Rats 735 open arms of the EPM negatively correlated with the number of box re-entries on the ET (r ¼ .271, p ¼ .026). The percentage of entries onto the open arms of the EPM also negatively correlated with ET box re-entries (r ¼ .266, p ¼ .028) and tended to correlate negatively with latency to exit the ET box (r ¼ .232, p ¼ .057). DISCUSSION The aim of this study was to investigate the ontogeny of anxiety-like behavior in rats from adolescence to adult- hood. The results indicated that the amount of exploratory behavior and measures of anxiety-like behavior decreased across the age groups on all three behavioral tests. More speciﬁcally, the amount of locomotion in the OF, as measured by line crossings or distance moved, increased from adolescence to adulthood; the amount of time on the open arms and number of open arm entries in the EPM increased from adolescence to adulthood; and older adults made the fewest re-entries into the start box in the ET. Our results are in line with previous reports of lower levels of exploration by adolescents in the OF (Candland & Campbell, 1962) and lower levels of open arm activity in the EPM compared to adults (Doremus et al., 2003). While animals at the beginning of adolescence (pnd 28) were not tested here, it is considered that the linear increases in exploration and anxiety-like behavior from adolescence to adulthood would hold, given previous research demonstrating lower locomotory and anxiety-like behavior in animals during early adolescence compared to their older adolescent counterparts (Hefner & Holmes, 2007; Lynn & Brown, 2009). In contrast, the results do not support the suggestion that adolescent rodents are more exploratory and less anxious than adults (e.g., Macri et al., 2002; Philpot & Wecker, 2008; Stansﬁeld & Kirstein, 2006), or the general conclusion that adolescent rodents are more risk-taking and respon- sive to novelty than adults (Spear, 2007). Here, we examine possible explanations for the differences between studies. One potential methodological difference between studies is whether the apparatus has been scaled for body size. Previous researchers have suggested that when comparing the behavior of adolescent and adult rodents, the apparatus should be scaled to take account of the size differences between age groups (Philpot & Wecker, 2008). Philpot and Wecker (2008) reported that adoles- cent male rats (pnd 31) spent a similar amount of time locomoting as late adolescents (pnd 56) in a scaled OF apparatus but reported that adolescents moved a greater total distance than the older animals. While our results also indicated that the amount of time spent locomoting did not differ with age, adolescents in our study moved a