Cyclical swelling and reddening of the circumgenital or sexual skin occurs during the menstrual cycle of several species of subhuman Primate. The changes vary considerably in extent, and in the rhesus macaque (Macaca mulatta), the most studied of all monkeys, swelling as a rule occurs only at puberty and during adolescence. Early accounts of the sexual-skin change in this species [Corner, 1923; Collings, 1926; Hartman, 1932] dealt with the condition in the adult, and, as was therefore to be expected, oedema was not considered to play any part in the change. Observations on the baboon, however, a species in which swelling is very prominent, showed that it represented a true oedema, and suggested that the swelling is due to transudation of fluid from the capillary bed into the intercellular spaces [Zuckerman, 1930]. Since it was known at the time that sexual-skin changes are due to oestrogenic action [Allen, 1927; Hartman,