Renal blood flow was measured in rats anaesthetized with pentobarbital sodium by employing the PAH clearance method and the 133Xe washout technique. Both methods were in good agreement. In 17 rats a 25% decrement of blood pressure was attained through haemorrhage: RBF measured by PAH clearance declined by 3%, according to 133Xe washout by 2.7%. In futher 21 rats an increase of blood pressure by an average of 26% was achieved by occlusion of the carotid arteries; RBF measured by PAH clearance declined by 3% according to 133Xe washout by 1.1%. RBF was therefore almost perfectly autoregulated during both experimental manoeuvres: glomerular filtration rate (inulin clearance) was autoregulated in a similar manner. Water and sodium excretion changed in proportion to changes of blood pressure.