The rhesus monkey (M. mulatto) has become the laboratory animal of choice for study in the physiology of reproduction because of the 28 day menstrual cycle, an extended pregnancy (circa 168 days), and the fact that the genital tract, anatomically, is so like a miniature replica of the human. In mating these monkeys, the routine procedure was to place the 9, on the 11th day of the menstrual cycle, in the cage of a cf for a period of 48 hrs. The results of 5 yrs., 1939 through 1943, have shown that in each year roughly 1/3 of the pregnancies followed a single mating. In 1944, the initial mating day was delayed to the 17th day of the menstrual cycle. Twenty-five animals were mated more than an avg. of 3 times each, 80 times in all, but at the end of 4 mos. only 1 pregnancy had been effected. In contrast, when the remaining 24 monkeys then were mated on the 11th day of their next cycle, 7 monkeys became pregnant. The menstrual record of the animal which became pregnant after a 17th day mating showed atypically long cycles of 48, 48 and 37 days. It was concluded that mating opportunity on the 17th day of the menstrual cycle is too late in relation to the time of ovulation and viability of the ovum. The 11 -13th days of the menstrual cycle remain the preferred days for mating in this animal, whose menstrual cycle normally has a model length of 28 days.