So little is known of the reproductive processes of any South American monkey that additional data on the typical genus Cebus are worthy of record. Capuchins seldom breed in captivity, only three records during a hundred years being given for the London Zoo by Zuckerman1 and no further cases being mentioned in his revised report of 19372. Since that date Hamlett3 has given a detailed account of the œstrous cycle, ovulation and menstruation in Cebus, but he left many questions unanswered.