Peak values in the karyopyknotic index of vaginal smears from the squirrel monkey occurred at intervals of 10.9±0.1 days. The urinary excretory rate of a metabolite similar to preg-nanediol increased when ovulation was induced by exogenous gonadotropin therapy. Small amounts of this metabolite were excreted during lactation, periods of anoestrus and during the follicular phases of normal oestrous cycles. Significantly larger amounts were excreted during the 157 days gestation period and during the luteal phase which persisted for one week after a peak karyopyknotic index value was observed. Changes in the pattern of urinary electrolyte excretion were not correlated reliably with changes in ovarian differentiation. Sexual motivation was greatest at mid-cycle, became less after conception and persisted at a low level throughout gestation and lactation. The occurrence of oestrus was seasonal and independent of physical contact with males. The development of secondary sexual characteristics in the male was synchronized with the onset of seasonal oestrus in the females.