than the 15 min we used in the other experiments. In this task there was again no significant genotypic difference in corticosterone levels that resulted from free exploration of the novel compartment (HEB, 8.6 ± 1.6 μ/dl; LEB, 6.4 ± 1.0 μg/dl; not significant). After longer familiarization, these intermediate corticosterone values appeared to reflect the expected reduction in stress (36). Both genotypes showed an aversion to the novel compartment: HEB mice spent 405 ± 116 sec of the possible 1,200 sec in the novel compartment, and LEB spent a comparable 490 ± 75 sec there. These responses are similar to some inbred strains tested in the free exploratory procedure previously reported (17, 25, 26), suggesting that the relative degree of familiarity with an apparatus can have a large effect on the degree of novelty preference observed. Clearly, the behavioral differences between HEB and LEB mice cannot easily be explained on the basis of differences in stress responses to the testing situation and furthermore demonstrate that stress and exposure to novelty are related in most test situations.
