An attempt of granulosa cell tumor (GCT) induction by prolonged administration of exogenous pregnant mare’s serum gonadotropin (PMSG) to young, mature, and middle-aged Balb-C mice resulted mainly in stromal luteal cell proliferation. 48.9% of young mice, 64.5% of mature mice and 65% of middle-aged mice developed luteal cell proliferation. This effect seemed to be dependent on duration of treatment, mainly in young mice. Young mice injected for less than 6 months developed significantly less luteal cell proliferation than those injected for more lhan 6 months. Only 1 GCT was found. This study seems to support the contention that besides gonadotropin stimulation, other factors may be involved in GCT development. A large number of PMSG-injected mice developed generalized lymphosarcoma, the mechanism of which remains unknown.