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T he hemodynamics of the rat kidney were studied during reduction of renal arterial pressure to 35-40 mm Hg (H), and after volume expansion at that pressure with 0.9% NaCl (IS), 1.7% NaCl (HS), 5% mannitol in 0.9% NaCl (MS), 5% mannitol in water (MW), or 50 mM mannitol + 125 mM NaCl. During H, left renal blood flow (RBF) was 0.8±0.1 ml/min. Expansion with IS did not alter RBF, but expansion with HS, MS, MW, and 50 + 125 mM NaCl elevated RBF to 200-250% of hypoperfusion values. Glomerular capillary pressure rose significantly from 15.7±0.7 mm Hg during H to 22.3±1.1, 24.4±0.7, and 26.6±0.7 mm Hg following expansion with HS, MS, or MW, respectively. Efferent arteriolar pressure also rose significantly to 6.9±0.5, 9.7±0.8, and 9.5±0.9 mm Hg, respectively. Preglomerular resistance fell to 18-24% of H values, and postglomerular resistance fell to 58-74% of H values after expansion with HS, MS, or MW. Glomerular filtration (GFR) could not be detected during H or after IS expansion. HS and mannitol-containing solutions restored GFR to 0.10±0.02-0.15±0.02 ml/min, and single nephron glomerular filtration to 6-12 nl/min. Papaverine, acetylcholine, and kinins had no effect on RBF or GFR at a perfusion pressure of 35-40 mm Hg. We conclude that mannitol and HS have the capacity to augment RBF during hypoperfusion by reducing arteriolar resistance. The mechanism of the rise in RBF is uncertain; it may be due to changes in effective osmolality of the extracellular fluid or to a direct action of mannitol on vascular smooth muscle. Other potent vasodilators were ineffective during hypoperfusion. Restoration of GFR occurs as a result of the combined effects of augmented RBF and elevated net filtration pressure.