Inhibition of corpus lutea progesterone synthesis by alkaloids in cigarette smoke might, in part, explain the generally poorer outcome of pregnancy in women who smoke. The present experiments evaluated the effects of alkaloids in cigarette smoke on progesterone biosynthesis and cell viability. Studies were initiated using primary cultures of human granulosa cells. Incubation of the granulosa cells with nicotine, cotinine, anabasine, the combination of nicotine, cotinine and anabasine, or an aqueous extract of cigarette smoke resulted in inhibition of progesterone synthesis. The alkaloids and smoke extract decreased the DNA content of the culture dish. These findings suggested a cytotoxic effect of the alkaloids. Growth curves conducted using the gonadotropin-responsive, progesterone-synthesizing MA-10 cell line confirmed growth inhibition by the alkaloids and smoke extract. Together, these data suggest that cigarette alkaloids inhibit cellular progesterone synthesis both by inhibiting progesterone synthesis and by causing less-specific toxic effects to the cell.