The induction of follicular growth, ovulation, and atresia by heterologous gonadotropic preparations was studied late in the reproductive cycle of the adult female guinea pig. Human chorionic gonadotropin (HCG) administration (10 IU) 12 days following the first signs of opening of the vaginal membrane was found to stimulate ovulation within 24 h in all animals studied, as evidenced by recovery of ova from their oviducts as well as the presence of postovulatory follicles in their ovaries. Histologically, ovaries of animals receiving HCG exhibited atretic changes in most of the follicles smaller than 999 micrometer in diameter. Pregnant mares serum gonadotropin (PMSG, 10 IU) administered on days 9 and 10 of the cycle was not sufficient to stimulate ovulation in this species although histological changes in the follicular complement were observed. Administration of PMSG prior to the HCG appeared to have an inhibitory effect on ovulation induction. Follicles luteinizing with entrapped ova were seen in all groups receiving exogenous gonadotropin, although they were most prevalent in the animals receiving the maximum total gonadotropin doses (i.e. PMSG + HCG).