Four adult male rhesus monkeys formed a new social group with 13 adult females. The male who became dominant [alpha] showed a progressive increase in plasma testosterone. The male who became subordinate to the other three males showed an 80% fall in testosterone from baseline levels. After 7 weeks, this group was introduced to a well-established breeding group, and all four males became subordinate to all members of the breeding group. All four males evidenced a fall in testosterone during the first week after introduction, and within 6 weeks their levels were approximately--80% of pre-introduction values. The alpha male of the breeding group showed a large increase in testosterone [238%] 24 hr after he successfully defended his group and became the dominant animal of the larger, newly-formed group. Thus, plasma testosterone levels appear to be significantly influenced by the outcome of conflict attendant to alterations in status of rhesus monkeys living in social groups.