Successful induction of ovulation in some cyclic adult and prepubertal species of Old World monkey, Macacca mulatta, has been reported by Van Wagenen & Simpson (1957) and by Simpson & Van Wagenen (1962), using injected extracts of monkey pituitaries and human postmenopausal gonadotrophins respectively. In these studies ovulation was determined by histological examina- tion of the ovary and no attempts were made to recover freshly ovulated eggs from the Fallopian tubes. Efforts to induce ovulation by injection of monkey pituitary were unsuccessful. The first experiments are now reported of inducing ovulation, with injections of pregnant mares serum (pms) and human chorionic gonadotrophin (hcg), in a species of New World monkey, Saimiri sciureus, the common squirrel monkey, as a step towards obtaining pregnancies in this species by artificial insemination. It was impossible to obtain accurate information as to the stages of the repro- ductive cycle in these monkeys from examination of vaginal lavage and the injections of gonadotrophins were thus given without reference to the timing of their reproductive cycles; in contrast to the work of Van Wagenen & Simpson (1957) and Simpson & Van Wagenen (1962). The pms (Pabryn\p=m-\Paines& Byrne Ltd) was injected subcutaneously into the lumbar region of mature females (600 to 700 g body weight) as a range of doses in distilled water, given twice daily (09.00 hours and 16.00 hours) for 9 consecutive days. During the last 4 days of these injections, the monkeys also received either 250 i.u. or 500 i.u. of hcg (Pregnyl\p=m-\Organon)given concurrently with the pms. Several methods were used to observe ovulation and recover eggs from the Fallopian tubes after cessation of dosing. Initially, the ovaries were observed for fresh sites of ovulation by laparotomy on the afternoon of Day 11 or Day 12 from the commencement of dosing, using 0\m=.\25ml Nembutal (60 mg/ml Abbott Laboratories) anaesthetic given intraperitoneally. In some monkeys, attempts were made to flush the eggs from the Fallopian tubes with normal saline while in