Plasma levels of follitropin (FSH), lutropin (LH) and testosterone (TE) were estimated in 5 anaemic haemodialyzed patients before and after 3 months of erythropoietin treatment (EPO group), in 5 male haemodialyzed patients with a haematocrit value of 33% (which was of the same magnitude as the post-treatment haematocrit value in the EPO group) and in 15 male healthy subjects. After EPO treatment, haematocrit values rose from 23.0 ± 0.9 to 34.6 ± 0.75%. EPO treatment induced a significant suppression of basal plasma FSH and LH levels, while plasma TE levels slightly increased. After EPO treatment, the response of plasma FSH and LH to luliberin administration was significantly reduced. As the endocrine profile of EPO-treated patients differed from that of haemodialyzed patients showing a similar haematocrit value, it seems that EPO-induced hormonal changes are not or not only related to improvement of anaemia. Normalization of the pituitary-gonadal feedback in EPO treatment seems to participate in the pathogenesis of improved sexual activity in these patients.