The action of Gly-Tyr-NH

2

（GY-NH

2

） and Gly-Tyr-LYS（GYK） on ￣（125）I-LH binding

cAMP accumulation and progesterone production was investigated. Incubation of rat luteal cells for 2.5 h with GY-NH

2

and GYK at dosage of 0. 2 mmol/L caused a significant inhidition of basal and gonadotropin-stimulated steroidogenesis. GY-NH

2

and GYK were also found to reduce cAMP formation in response to hCG. The activity of adenylate cycles of luteal cells was inhibitd by 0. 2 mmol/LGY-NH

2

and GYK. GY-NH

2

and GYK at a concentration of 0. 2mmol/L were not found to have an inhibitory effect on 8Br-cAMP-stimulated progesterone preduction. GY-NH

2

and GYK did not affect ￣（125）I-LH binding to LH receptors on the luteal cell surface. These results suggest that GY-NH

2

and GYK inhibit steroidogenesis at the step of gonadotropin-stimulated cAMP formation in luteal cells. Adenylate cyclase in luteal cells was also inhibited.