Plasma samples were obtained from 8 conscious rhesus monkeys at 3-4 day intervals throughout the menstrual cycle. Oxytocin concentrations were significantly higher in mid-cycle (days 10-11) than at days 4-5 (P < 0.01) and days 28-29 (P < 0.02, Wilcoxon signed rank test). No significant correlations between oxytocin and estradiol or progesterone were found.