Gonadotropins were used to stimulate luteal function, as determined by progesterone secretion, in both in vitro and in vivo systems. LH and hCG were capable of significantly stimulating progesterone secretion in the in vivo systems. Stimulation of progesterone secretion by hCG was greater than that for LH. PMSG failed to increase progesterone production at any level of treatment. hCG was also used to stimulate progesterone production by the corpus luteum in mares during early gestation. hCG administration resulted in a significant (p < 0.10) increase in peripheral progesterone levels in treatment mares through day 14 post-estrus. Peripheral progesterone concentrations were also higher in hCG treated mares for days 15 through 30 post-estrus in mares that conceived. hCG treatment had no influence on anterior pituitary release of LH.