The effect of female sex hormones on nitric oxide (NO) production was studied in alveolar macrophages (AMs). Male rats were treated with endotoxin (LPS) intratracheally or saline as control. AMs were obtained by bronchoalveolar lavage 90 min later and were cultured in the presence or in the absence of LPS and 17beta-estradiol or progesterone (10(-9) to 10(-4) M). NO production was assessed by measurement of nitrites in the medium. In some experiments, NO production by AMs was measured in intratracheally LPS-treated orchidectomized rats or in female control and ovariectomized rats. Both spontaneous and stimulated NO production were higher in AMs from female than from male rats, but without statistical significance. However, ovariectomy induced significant inhibition in spontaneous production of NO by AMs. In orchidectomized rats, the NO response by AMs to LPS stimulation relative to spontaneous NO production was significantly downregulated. Female sex hormones in physiological concentrations seem to be necessary for spontaneous NO production in female rats. Pharmacological doses of estradiol inhibited in vitro LPS-stimulated NO production in AMs of both saline- and LPS-treated rats, and basal NO production only in LPS-treated male rats. Progesterone at 10(-4) M inhibited basal and in vitro LPS-stimulated NO generation by AMs of both saline- and LPS-treated male rats. In LPS-treated female rats in vitro LPS-stimulated NO production was not affected by estradiol treatment. In ovariectomized LPS-treated female rats progesterone at 10(-5) M significantly inhibited NO production by in vitro-stimulated AMs. Thus female sex hormones may contribute to the gender-related differences in the immune response.