The circadian rhythm (CR) of plasma renin activity (PRA), plasma aldosterone (PA), and plasma Cortisol (PC) was investigated in 8 patients with kidney transplantation, and in 10 patients with heart transplantation. Ten clinically healthy subjects were studied as controls. The transplanted patients were all under cyclosporine treatment associated with prednisone (PDN). Time-qualified levels of PRA and PA were seen to be higher than normal in both groups of transplanted patients. The analysis of PRA and PA circadian rhythm provided evidence for a systematically higher level of within-day concentrations. The higher level of oscillation suggests the occurrence of a condition of hyperreninemic hyperaldosteronism. The higher levels of PRA and PA 24-h values show no periodicity. The finding suggests the abrogation of the rhythmic function for renin-aldosterone system. The disappearance of PRA-PA circadian rhythm seems to be attributable to a side effect of immunosuppressive therapy.