A decrease in serum progesterone at the end of pregnancy is essential for the induction of parturition in rats. We have previously demonstrated that LH participates in this process through: 1) inhibiting 3β-hydroxysteroid dehydrogenase (3β-HSD) activity and 2) stimulating progesterone catabolism by inducing 20α-hydroxysteroid dehydrogenase (20α-HSD) activity. The objective of this investigation was to determine the effect of LH and progesterone on the luteal expression of the steroidogenic acute regulatory protein (StAR), cytochrome P450 side-chain cleavage (P450_scc), 3β-HSD, and 20α-HSD genes. Gene expression was analyzed by Northern blot analysis 24 and 48 h after administration of LH or vehicle on Day 19 of pregnancy. StAR and 3β-HSD mRNA levels were lower in LH-treated rats than in rats administered with vehicle at both time points studied. P450_scc mRNA levels were unaffected by LH. The 20α-HSD mRNA levels were not different between LH and control rats 24 h after treatment; however, greater expression of 20α-HSD, with respect to controls, was observed in LH-treated rats 48 h after treatment. Luteal progesterone content dropped in LH-treated rats at both time points studied, whereas serum progesterone decreased after 48 h only. In a second set of experiments, the anti-progesterone RU486 was injected intrabursally on Day 20 of pregnancy. RU486 had no effect on 3β-HSD or P450_scc expression but increased 20α-HSD mRNA levels after 8 h treatment. In conclusion, the luteolytic effect of LH is mediated by a drop in StAR and 3β-HSD expression without effect on P450_scc expression. We also provide the first in vivo evidence indicating that a decrease in luteal progesterone content may be an essential step toward the induction of 20α-HSD expression at the end of pregnancy in rats.