We investigated the effect of nicotine and its methylated metabolite, N-methyl-nicotine (M-nicotine), on human luteal cells by measuring release of progesterone and prostaglandins (PGs) from cultured cells and by testing gene expression of vascular endothelial growth factor (VEGF), an angiogenic factor strictly involved in luteal pathophysiology. Primary cultures of human luteal cells were treated for 24 h with nicotine and M-nicotine (from 10⁻⁶ to 10⁻¹¹ M) either alone or combined with hCG (25 ng/ml); progesterone and PGs were assayed in the culture medium. In another group of experiments, luteal cells were treated for 24 h with nicotine and M-nicotine (10⁻⁷ M) to perform reverse transcriptase-polymerase chain reaction on VEGF mRNA. Nicotine and M-nicotine negatively affected basal luteal steroidogenesis at all tested concentrations, but neither was able to affect hCG-induced progesterone release. Both substances were able to significantly increase PGF_2α release from luteal cells, with a dose-related efficacy for M-nicotine. On the contrary, PGE₂ release was significantly inhibited by both nicotine and its metabolite. Finally, nicotine was able to increase VEGF mRNA expression significantly, whereas M-nicotine was not. In conclusion, nicotine and M-nicotine can induce a sort of luteal insufficiency by inhibiting progesterone release, probably through modulation of the PG system.