Effect of angiotensin converting enzyme inhibitor (ACEI) and calcium channel blockers (CaB) on renal blood flow (RBF), glomerular filtration rate (GFR), and autoregulation (AR) of RBF were studied on the uninephrectomized spontaneously hypertensive rat (SHR) under the conditions of high (H) and low (L) salt loading. SHR was given with 0.9% or 0.09% NaCl solution as drinking water (GH, GL). Each group was divided into three groups for treatment with enalapril (Enp) and nitredipine (Nit); i.e. Enp group (GHE, GLE), Nit group (GHN, GLN) and control group (GHC, GLC). After 6 weeks, inulin clearance (Cin) was determined and RBF was measured by means of an electromagnetic flow meter. The renal arterial pressure was lowered by clamping and changes in RBF and AR were examined. Cin showed higher values of 1.85 and 1.69 ml/min in GHN and GLN, as compared to be 1.33 and 1.28 ml/min in GHC and GLC (p less than 0.01). Filtration fraction (FF) showed lower values of 0.18 and 0.20 ml/min in GHE and GLE (p less than 0.01), whereas 0.29 and 0.30 in GHC and GLC respectively. RBF was markedly lower at 7.4 ml/min in GLC as compared to 9.9 ml/min in GHC (p less than 0.01). In GH, GHE showed a higher value of 11.6 ml/min, as compared to GHC (p less than 0.01). In GL, comparing with GLC the value was much higher of 12.1 ml/min in GLE (p less than 0.01). AR of RBF diminished in GLC at higher blood pressure as compared to GHC (p less than 0.01). It was maintained at lower blood pressure in GLE (p less than 0.01), but there were no significant differences between four groups; i.e. GLN, GHC, GHE and GHN. In summary, low salt loading reduced RBF and suppressed AR. Enp elevated RBF, lowered FF and caused AR to be maintained even at lower blood pressure. Nit elevated RBF and GFR without changing FF, and did not suppress AR. These results indicate that, in hypertension complicated with moderate dysfunction, both ACEI and CAB are expected to exhibit the beneficial effects on maintenance of renal circulation, despite though the different mechanism.