The integrity of renal haemodynamics is essential for the maintenance of normal renal function in the normotensive and hypertensive state. Systemic and renal haemodynamics have been determined by the radioactively labelled microspheres technique in normotensive, Goldblatt I, Goldblatt II and deoxycorticosterone (DOC)-saline-treated hypertensive rats. Cardiac output was similar between normotensive and hypertensive rats, while total peripheral resistance was similarly higher in all hypertensive animals when compared to normotensive animals. Renal blood flow per weight unit was similarly decreased in all hypertensive animals (3.77 ml/min . g (S.D. 0.85, n = 11); the unclipped kidneys of Goldblatt II rats, 3.76 ml/min . g (S.D. 0.89, n = 11); the clipped kidneys of Goldblatt II rats, 3.62 ml/min . g (S.D. 0.78, n = 9); the kidneys of Goldblatt I rats and 2.95 ml/min . g (S.D. 0.83, n = 10) the kidneys of DOC-saline rats). It is concluded that kidneys from experimentally hypertensive rats fail to preserve normal renal blood flow, even at elevated arterial blood pressures, regardless of the pathophysiology of hypertension.