The steroidogenic effects of bovine luteinizing hormone (lh) and human chorionic gonadotrophin (hcg) in vivo were investigated by measuring the concentration of progesterone in peripheral plasma as an index of luteal secretory activity. The gonadotrophins were injected intravenously on Days 5, 10, 15 and 20 of the oestrous cycle, and serial samples of peripheral blood obtained before and after the injections. The steroidogenic response to lh or hcg was variable. Statistically significant increases in plasma progesterone levels were produced in five out of ten experiments; the magnitude of this increase was less than 100% in three cases and the duration of stimulation was less than 3 hr in four cases. The life-span of the corpus luteum was prolonged and new ovulations were induced by the gonadotrophins.