Injections of 1 mg PGI2 directly into the bovine corpus luteum significantly increased peripheral plasma progesterone concentrations within 5 min. Concentrations were higher in the PGI2-treated heifers than in saline-injected controls between 5 and 150 min and at 3.5, 4, 5, and 7 h post-treatment. Levels tended to remain elevated through 14 h. Saline and 6-keto-PGF1 alpha were without effect on plasma progesterone levels. The luteotrophic effect of PGI2 was not due to alterations in circulating LH concentrations. An in vitro experiment assessed the effects of either PGI2 alone or in combination with LH on progesterone production by dispersed luteal cells. Progesterone accumulation over 2 h for control, 5 ng LH, 1 microgram PGI2, 10 micrograms PGI2, and 10 micrograms PGI2 plus 5 ng LH averaged 99 +/- 42, 353 +/- 70, 152 +/- 35, 252 +/- 45, and 287 +/- 66 ng/ml (n = 4), respectively. Thus PGI2 has luteotrophic effects on the bovine CL both in vivo and in vitro.