Hypophysectomy, at any stage of pregnancy, interrupted gestation in the hamster. Pituitary removal at day 1, 4 or 8 of pregnancy resulted in rapid regression of the corpus luteum (CL), whereas after day 12, when the placenta has been established, the histologic integrity of the CL was maintained. After hypophysectomy at day 4 of pregnancy, the daily injection of prolactin, but not FSH or LH, maintained the CL as a histologic entity, but pregnancy was terminated. Treatment with prolactin and FSH increased the vascularity of the CL and maintained pregnancy in the majority of animals; hence, the concerted interaction of these 2 hormones constituted the minimal luteotropic complex (LtH) of the hamster. Evidence is presented that the activity of FSH cannot be attributed to contamination with LH. The addition of exogenous LH to the LtH complex had dose-dependent effects in hamsters hypophysectomized on day 4 of pregnancy. High daily doses of LH (50–100 μg) interrupted pregnancy in all animals and led to a marke...