There have been no previous reports on the successful artificial insemination of New World primates. Artificial insemination has now been achieved in the squirrel monkey (Saimiri sciureus) using semen recovered by rectal electro-ejaculation from fertile males. The growth of ovarian follicles was stimulated in five adult female monkeys, weighing 600–700 g., with twice-daily s.c. injections of 20 i.u. pregnant mare serum (PMS) (Pabyrn, Paines and Byrne Ltd.) given for 9 consecutive days. During the last 4 days the monkeys also received twice-daily s.c. injections of 250 i.u. human chorionic gonadotrophin (HCG) (Pregnyl, Organon). All gonadotrophin injections were given in distilled water into the lumbar region of the monkeys. No attempt was made to synchronize the injections with a particular time of the oestrous cycle. Semen was obtained by electro-ejaculation in three male squirrel monkeys of proven fertility, using a bipolar rectal probe (Bennett, 1967). The ejaculates (0·5 ml.) were collected