In a human conception cycle, the expected decline in progesterone production by the corpus luteum during the late luteal phase is prevented by human chorionic gonadotrophin (HCG) secreted by the implanting blastocyst. This study investigated the expression of components of the synthetic pathway for progesterone in human corpora lutea in the presence and absence of HCG in vivo. Corpora lutea were obtained from: (i) normally cycling women at the time of hysterectomy and classified on the basis of the urinary luteinizing hormone (LH) surge as early (n = 3), mid- (n = 3), or late luteal (n = 3); or (ii) women who had received daily doubling doses of HCG (n = 3) to 'rescue' the corpus luteum. Expression patterns of steroidogenic acute regulatory protein (StAR), cytochrome P450 cholesterol side-chain cleavage (P450scc) and 3beta-hydroxysteroid dehydrogenase (3beta-HSD) were investigated by Northern blotting, in-situ hybridization and immunohistochemistry. Luteal 'rescue' with HCG was associated with the continued expression of these components. In the late luteal phase, in the absence of HCG, expression remained but was more variable. The expression of 3beta-HSD mRNA was significantly reduced during the luteal phase (P<0.01). In conclusion, during luteal 'rescue', HCG acts to maintain the steroidogenic pathway. In the absence of HCG, the decline in progesterone production begins in the presence of the main components of the steroidogenic pathway. While unlikely to initiate this decline, the altered expression levels of these components, particularly that of 3beta-HSD, may contribute to the continued reduction in progesterone production.