In 7 patients with end stage renal failure, anterior pituitary function was tested by simultaneous application of maximally effective doses of the hypothalamic releasing peptides, corticotropin-releasing hormone, growth hormone-releasing hormone, thyrotropin-releasing hormone and gonadotropin-releasing hormone, and compared to 8 normal controls. In addition to the pituitary hormones, plasma cortisol, thyroxine and testosterone concentrations were measured. To test for possible effects of treatment with recombinant human erythropoietin (rhu-EPO), all patients with chronic renal failure were studied again after partial correction of anemia by treatment with erythropoietin. Before initiation of rhu-EPO treatment, plasma concentrations of follicle-stimulating hormone were significantly elevated and the thyroid-stimulating hormone and prolactin responses to thyrotropin-releasing hormone blunted when compared to normal controls. Treatment with rhu-EPO induced a significant increase in plasma ACTH and follicle-stimulating hormone concentrations. All other pituitary functions remained unchanged. Thus, the general improvement in well-being, working capacity and sexual activity cannot be attributed to hormonal changes.