The present investigation was carried out on 17 male patients, 2 of whom nephrectomized, affected by terminal renal failure on regular 4 h dialysis three times a week. Plasma prolactin (PRL), aldosterone (PA) and renin activity (PRA) were determined by radioimmunoassay before, and after the 1st, 2nd and 3rd h of hemodialysis. High levels of PRL were found in 9 nonnephrectomized patients; during the 1st and 2nd h of dialysis means values of PRL showed a slight tendency to increase, whilst during the 3rd h values decreased to predialysis concentrations. In one case a fall in PRL levels immediately after nephrectomy was observed but in another patient who came to our attention after surgery, PRL was significantly increased with values higher than those usually found in nonnephrectomized patients. PRA was elevated in 10 patients and showed no significant modifications during hemodialysis. PAC was high in 8 patients and decreased significantly during hemodialysis. A drop in sodium and potassium levels and a decrease in body weight during dialysis were also observed. These results suggest that in uremia the high levels of PRL are not consequent to hydroelectrolyte disorders or to a condition of hyperaldosteronism, since these parameters present a different pattern during hemodialysis.