It has been shown that plasma corticosterone level in isolated male albino rats was considerably lower than in grouped rats. Plasma corticosterone level in grouped animals increased during wakefulness and diminished in the sleepy state. Dominance relations were studied by observing the ‘spontaneous’ behaviour of rats kept in groups of ten for 4 to 25 weeks. The ratio of total number of encounters won to total lost by each rat was used to rank the rats. Linear relationships between social rank and plasma corticosterone level could not be established. However, the corticosterone level in the most submissive rats was twice as high as in dominant animals. Formalin administration to grouped rats provoked a less marked reaction of the pituitary-adrenal system than in isolated animals.