The incidence of cyclosporine A (CsA) nephrotoxicity with reference to the temporal stage of administration was studied during a chronic 21-day treatment in male Wistar rats. Oral administration (20 mg/kg/day) was given at four different times: 1, 7, 13, or 19 hours after light onset (HALO). Plasma creatinine and blood urea nitrogen (BUN) levels were determined at regular intervals over the 24 h: before treatment (day 0); 7, 14, and 21 days after the beginning of treatment (days 7, 14, and 21); and 7 and 14 days after CsA withdrawal (days 28 and 35). At the same times, creatinine clearance and g-glutamyl transferase urinary excretion were determined in the groups of animals treated at 7 and 19 HALO. Residual concentrations of CsA in the renal tissue were measured at the end of the treatment period (day 21) in all groups. Nephrotoxicity of CsA was dependent on the temporal stage of administration. The renal vasoconstriction showed by the increase in plasma creatinine and BUN levels and the decrease in creatinine clearance was maximal when the CsA was given at 7 and 19 HALO and was correlated to the tissue concentrations of CsA. Tubular injury seems to occur earlier and the return to normal function less rapidly in animals treated at 19 HALO compared with animals treated at 7 HALO.