Physiological and behavioral consequences of formation of female-female relationships were investigated in adult squirrel monkeys. Plasma cortisol, heart rate, and behavior were evaluated during confinement in a test cage while animals were housed alone, during the first hour of isosexual pair formation, and while animals were housed as isosexual dyads. In addition, basal cortisol levels were assessed both before and after pair formation, as were behavioral and cardiovascular responses to social and nonsocial stimulus presentations. Basal cortisol levels underwent a marked and sustained reduction following formation of pairs, independent of both dominance status and the quality of social interactions between pairmates. In contrast, adrenocortical responsiveness was not altered by social conditions, apart from a modest and transitory difference between dominant and subordinate females in their initial response to pair formation. Heart rate declined sharply during each test session, but did not reliably differentiate social conditions, stimulus conditions, or dominance status. These findings contrast with results of a parallel study of male squirrel monkeys and suggest that isosexual relationships in males and females are associated with different sociophysiological processes.