Cycling hamsters injected with selected doses of PMS displayed a dose—response relationship in the number of eggs ovulated. Cycle length was unchanged following a single injection of 5 to 30 IU PMS on day 1 of the cycle, but was prolonged to 5 days on the injection of 60 IU PMS. All doses of PMS increased peripheral E1 and E2 above levels normally found in untreated cycling hamsters. With 30 IU PMS there was about a 4–fold increase in E1 levels and about a ten—fold increase in E2. Estrogen (E1 and E2) levels returned to control values in the 5 and 10 IU PMStreated groups by proestrus (day 4), but remained elevated in the IS and 30 IU treated groups. (Endocrinology 92: 1547, 1973)