Ninety-three female cynomolgus monkeys were monitored throughout 647 menstrual cycles; 93.2% of these cycles were 22--37 days long and were normally distributed, with a mean of 29.2 days. Menstrual bleeding generally lasted for 3--5 days and was not related to the length of the cycle. The levels of progesterone and oestradiol-17beta in the plasma were measured during the menstrual cycle in 30 monkeys. The concentration of oestradiol-17beta reached a mid-cycle peak on day 11 or 12 of the cycle. The interval between the beginning of the cycle and the oestrogen peak was constant; the interval between the oestrogen peak and the end of the cycle increased or decreased with the cycle length. The level of progesterone began to rise at about the time of the oestrogen peak and remained raised for longer periods as the length of the cycle increased. The length of the menstrual cycle, therefore, appeared to be determined by the duration of the increase in the level of progesterone or by the life of the corpus luteum in the luteal phase. The relationship between the lengths of the cycle and the luteal phase can be defined by the expression: cycle length = 12.6 + 0.96 X length of luteal phase (correlation coefficient = 0.875).