The ability of daily oxytocin injections to inhibit the growth of the corpus luteum and cause precocious estrus in the bovine was used as a tool to test the luteotropic properties of various gonadotropins and growth hormone. Corpora lutea were collected from each of 32 Holstein heifers at 11 days post estrus after the following treatments: (1) daily oxytocin injections (15 USP units/cwt), (2) daily oxytocin plus gonadotropin or somatotropin injections or, (3) no treatment. Comparisons of weights, progesterone content, and cell types of these corpora lutea revealed that bovine STH (75 mg./day), ovine prolactin (1704 I.U. units/day), and two levels of equine LH (20 and 100 mg./day) were not luteotropic, since they did not overcome oxytocin inhibition. The means of total progesterone level in the corpora of six STH-treated heifers, five prolactin-treated heifers, and four low and five high level LH-treated heifers were 82 mcg., 21 mcg. 32 mcg., and 11 mcg., respectively. An average of 28 mcg. of total progesterone was found in corpora from heifers treated with oxytocin alone. In contrast five heifers receiving HCG (2000 IU/day) and five receiving a crude bovine AP extract (3.5 gland equivalent/day) produced corpora having mean total progesterone contents of 608 and 363 mcg., respectively. An average of 261 mcg. of total progesterone was found in corpora of untreated heifers. The results suggest that there is a specific luteotropic hormone in the bovine, and that it is not bovine STH, equine LH or ovine prolactin.