Hamsters were hypophysectomized (H in. Tables) at day 4 of pregnancy and injected with various gonadotropins over the next 4 days. Endpoints of luteal activity at day 8 included the maintenance of pregnancy, luteal size and luteal and plasma levels of progesterone measured by the competitive protein binding assay Following hypophysectomy, and replacement with saline, the corpora lutea (CL) were reduced to l/2Oth the size of intact controls and all parameters of luteal function were reduced to baseline levels. Ovine prolactin (PRL–1 mg) did not maintain pregnancy but the CL were increased to 1/2 of normal size and had l/8th the progesterone content of CL from intact hamsters; plasma levels of progesterone, however, were the same as the saline injected group. When 1 mg PRL and 200 μg ovine FSH were injected concurrently, all indices of luteal activity were restored to control levels. Hence, this treatment constitutes the minimal luteotropic complex. The best results were obtained with 1 mg PRL and 1–2 IU of pregnant mares serum. Substituting ovine LH (1–50 μg) for FSH in the luteotropic complex was ineffective in maintaining pregnancy and CL. Therefore, the luteotropic activity of 200 μg FSH is apparently not attributable solely to LH contamination. Daily doses of 10 μg LH or more increased the concentration of progesterone in non—luteal tissue and 25 μg or more of LH led to interstitial hypertrophy. There was no evidence for synergism when 0.1-1 μg LH was combined with the luteotropic complex and large doses of LH (10–50 μg) interrupted pregnancy in the majority of animals. The minimal effective dose of the complex was 0.1 mg ovine PRL and 100–200 μg ovine FSH. With this regimen, embryonic swellings were considerably smaller than controls at day 8 of pregnancy and this was associated with the failure to differentiate antral follicles in the hypophysectomized animals. A single injection of estrogen plus the luteotropic complex restored the weight of the embryonic swellings to control values. This indicates that PRL and FSH while adequate to maintain progesterone secretion are insufficient to maintain estrogen secretion in the hypophysectomized hamster. (Endocrinology92: 235, 1973)