Plasma androgen (testosterone + dihydrotestosterone) and cortisol levels of fully mature wild-reared male squirrel monkeys were measured using radioimmunoassay and protein binding methods, respectively. A total of 337 plasma samples were collected over a 24-h period, and at weekly and bi-weekly intervals for a period of 13 months. Plasma levels of androgen (mean ±
se= 58.1 ± 4.0 ng/ml) and of cortisol (112.8 ± 4.8 μg/100 ml) are markedly higher in squirrel monkeys than in Old World monkeys. There were significant individual differences in plasma androgen levels. There was no apparent diurnal or annual cycle in plasma androgen levels. A group of 4 male squirrel monkeys housed in isolation cubicles had much higher levels of plasma cortisol (170.1 ± 7.7 μg/100 ml) than 10 non-isolated males (92.7 ± 4.3 μg/100 ml). There was a diurnal cycle in plasma cortisol for animals maintained in isolation cages, but non-isolates did not exhibit a diurnal pattern in plasma cortisol. There was no statistically significant correlation between plasma androgen and plasma cortisol levels; high levels of cortisol were not necessarily associated with low androgen levels. The variations in plasma androgen could not be attributed to any reliable cyclic phenomena, nor to stress, but may be due to maintaining animals under laboratory conditions.