We characterized renal tubular reabsorption before and during acute expansion in anesthetized 12-wk-old spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY). Although mean arterial pressure was higher in euvolemic, nondiuretic SHR than in WKY, 158 vs. 114 mmHg, kidney and nephron glomerular filtration rate (GFR) as well as fluid reabsorption by the proximal convoluted tubule, loop of Henle, and distal convoluted tubule-collecting duct were similar. In euvolemic SHR with aortic constriction (SHR-AC), an acute decrease in renal perfusion pressure to 114 mmHg reduced sodium and water excretion. Kidney and nephron GFR and fluid reabsorption by segments along the nephron resembled values for SHR and WKY. Infusion of isotonic saline (3 ml.100 g body wt-1.h-1) produced similar increases in fractional sodium and water excretion by SHR and WKY, whereas SHR-AC exhibited a blunted natriuresis and diuresis. During expansion, fluid reabsorption by the nephron segments did not differ appreciably among the three groups. The effect(s) of perfusion pressure on reabsorption by superficial nephrons may be covert and was not unmasked, or may be manifested preferentially by deeper nephrons. We conclude that kidneys of SHR require a higher arterial pressure than kidneys of WKY to excrete a given amount of salt and water.