The effects of longlasting dehydration on total and local renal blood flow and glomerular filtration rate have not been studied previously in the laboratory rat. We therefore determined zonal renal blood flow (ZRBF) in cortical and medullary zones by the 125I-iodoantipyrine and H2 gas washout techniques. After 8 days of water deprivation renal blood flow (RBF) was reduced by 65% and renal vascular resistance (RVR) had increased by 110%. Half the RVR increase could be ascribed to increased blood viscosity as reflected by increased hematocrit (42 to 60%). Fractional ZRBF decreased by 5% in the outer half and increased by 5% in the inner half of the cortex. Measurements of the relative single nephron glomerular filtration rate (sngfr) of superficial and deep nephrons, determined by the 14C-ferrocyanide technique, indicated no detectable changes in filtrate distribution. Both ZRBF and sngfr heterogeneity had increased in dehydrated as compared to control rats. In one rat studied with the H2 gas washout method, local ischemia and intermittent blood flow occurred. Similarly, cortical patches of nonfiltering nephrons were observed in another rat. These findings suggest that the increased heterogeneity of intrarenal flow and filtration may involve, and in part be due to, intermittent local changes in blood flow.