The purpose of this investigation was to assess the chronic effects of marihuana smoking on the basal and stimulated secretion of the pituitary hormones luteinizing hormone (LH), follicle-stimulating (FSH) and prolactin (PRL). Ten male chronic marihuana users and 10 age- and sex-matched healthy volunteers were studied by measuring hormone levels before and after i.v. administration of thyrotropin releasing hormone (TRH) and gonadotropin releasing hormone (GnRH). The basal and stimulated levels of LH were reduced in marihuana users, whereas FSH and PRL levels and responses were not different from the control subjects. The chronic use of marihuana may selectively impair the hypothalamic control mechanisms regulating LH seretion.