The occurrence of menstruation in female rhesus monkeys is difficult to assess reliably under field conditions, but quantitative data obtained in the laboratory (Hartman, 1932) have indicated that menstruation in this species is infrequent and irregular during the summer months; similar views have been expressed by others (van Wagenen, 1945; Corner, 1945; Kerber & Reese, 1969). However, Rowell (1963) found scant evidence of a summer period of amenorrhoea but reported that cycles were shorter in winter than in summer. Data on changes in the menstrual rhythm throughout the year tend to be confused by the interruptions caused by pregnancy and lactation, and we report here on annual changes in the occurrence of menstrual bleeding in female rhesus monkeys whose Fallopian tubes were ligated to prevent pregnancy. Thirty adult female rhesus monkeys (body wt 5·0–8·3 kg) were obtained from northern India and studied over a 6-yr period between May 1964 and