Human corpora lutea of pregnancy obtained at 5 to 11 weeks of gestation were incubatedin vitroin the presence of various concentrations of human chorionic gonadotropin (hCG) and prostaglandin F2α(PCF2α) Progesterone (P) and 17β-estradiol (E2) released into the medium during incubation were measured by radioimmunoassay. Pieces of the corpora lutea were also examined ultrastructurally before and after incubation. Release of P and E2into the medium was significantly increased by the addition of hCG and PGF2αin some cases. However, the response to hCG and PGF2aappeared to vary according to the age of the corpus luteum. Ultrastructurally, the lutein cells were well maintained with respect to the appearance of the smooth endoplasmic reticulum and mitochondria after 120-min incubations. On the basis of these observations, the following conclusions have been reached: 1) This incubation system seems to be suitable for investigating the direct effects of chemicals on lutein cellsin vitro.2) Both hCG and PGF2αdirectly stimulate steroidogenesis in the human corpus luteum of pregnancy, demonstrating their luteotropic actionsin vitro.3). Observed variations in P and E2secretion might have been due to the different activities of aromatizing enzymes at different stages of gestation.