Proximal tubular hydrostatic pressure (P_S) and subcapsular pressure i.e. renal interstitial pressure (P_S) were measured in isolated perfused kidneys of normotensive Wistar Kyoto rats (WKY) and of spontaneously hypertensive rats (SHR). At a renal perfusion pressure of 90 mmHg P_T and P_S were significantly lower in kidneys of SHR rats, while the fractional potassium and magnesium excretion rates were elevated. Superimposed venous occlusion of 15 mmHg increased P_T as well as P_S in both strains of rats but to the same leval, while the increase in fractional electrolyte excretion (sodium, potassium, chloride, calcium and magnesium) was again much more pronounced in kidneys of the SHR rats. Decapsulation of the kidneys of the WKY strain lowered the P_T values to that observed in SHR rats. Venous pressure elevation under these conditions yielded the same electrolyte excretion pattern in both types of kidneys. At comparable elevated P_T and P_S values in kidneys of the SHR rats the electrolyte excretion rates were much higher during venous occlusion than at increased perfusion pressure. The same experimental conditions in kidneys of WKY rats resulted in different values of P_T and P_S, whereas the electrolyte excretion rates were the same. The results strongly suggest that: 1) the absolute values of the intrarenal pressures are not correlated with electrolyte excretion pattern; 2) venous pressure elevation has a profound effect on electrolyte excretion rate only in SHR kidneys and in the decapsulated kidney of the WKY rat; 3) the capsular compliance in kidneys of SHR rats is higher.