The effect of housing conditions and the presence of a conspecific on corticoadrenal response to a novel environment was studied in male Sprague-Dawley rats. The response to a novel environment was the same in rats housed in groups and rats housed alone for 20 days before testing, suggesting that isolation during adulthood did not affect corticoadrenal response to a novel environment. The presence of a nonfamiliar rat (proceeding from another home cage) did not modify corticoadrenal response to the novel environment but the presence of a rat proceeding from the same home cage induced a higher corticoadrenal response to this stressful stimuli. These results can be explained considering that rats exhibit greater emotional reactivity, as measured by adrenal function, in situations which combine familiar (the other rat) and novel (the box) elements than those completely unfamiliar. © 1983 Academic Press, Inc.