Social influences on well-being in squirrel monkeys are being evaluated in animals housed with like-sex and/or opposite-sex companions. The assessment of well-being assumes as major criteria the ability to carry out basic biological functions in a normal fashion and the absence of persistent symptoms of stress or disease. Accordingly, the impact of social living arrangements on well-being is being evaluated using the following measures 1. Activity and reactivity in stress responsive systems, 2. Reproduction, and 3. Health status. Social living arrangements alter pituitary-adrenal activity and seasonal changes in gonadal hormone production. Males and females living with both same sex and opposite-sex companions exhibit a sustained reduction in basal adrenocortical activity. Living only with opposite-sex companions is not as effective in down-regulating this system. During the breeding season adrenocortical activity increases and midday basal values do not differ among animals in the various social conditions. Nonetheless, response to stress indicates that differences in pituitary-adrenal regulation persist even when basal values are the same. Seasonal weight increases are correlated with cortisol levels and are synchronized even among individually housed animals. Gonadal hormones increase follow cortisol and weight changes by approximately 1 month in females and socially housed animals, but individually housed males exhibit testosterone increases out of phase with the remainder of the colony and with their own cortisol and weight changes. Our findings indicate that there are multiple mechanisms whereby social stimulation may alter seasonality. Social cues that can be detected at a distance (auditory or olfactory) are sufficient to synchronize some seasonal transitions, whereas, others require direct interaction.