The present study was carried out to examine the effect of chronic dietary protein restriction on renal water handling in the rat. During hypotonic saline infusion, the malnourished rats showed a lower free-water clearance, corrected by inulin clearance (7.2 ± 0.4%), than normal rats (13.6 ± 2.5%, p < 0.051), although the fractional distal delivery of sodium did not differ from normal. Throughout hypertonic saline diuresis the free-water reabsorption (TcC<sub>H2</sub>o) corrected by inulin clearance was lower in malnourished rats (6.62 ± 0.64%) than in control animals (9.25 ± 0.62, p < 0.05). Moreover, when Tc<sub>H2</sub>o was referred to the osmolar clearance, malnourished animals showed lower values than normal. These results suggest a defect in NaCl transport in the thick ascending limb of Henle. In vitro measurements of diffusional water permeability (P<sub>DW</sub>) in the inner medullary collecting duct (IMCD) obtained from malnourished rats showed an increase from 40.0 ± 5.4 × 10<sup>-5</sup> cm/s to 71.3 ± 5.4 × 10<sup>-5</sup> cm/s by adding maximum effective concentration (50 μU/ml) of arginine vasopressin (VP) to the bath. These values were not different from the P<sub>DW</sub> observed in the IMCD of normal rats. In another series of microperfusion experiments, the hydraulic conductivity in IMCD of malnourished rats measured also in the presence of maximum effective concentration of VP was 29.7 ± 3.4 × 10<sup>-6</sup> cm/atm/s, a mean value not significantly different from that observed in the IMCD of normal rats (35.2 ± 4.3 × 10<sup>-6</sup> cm/atm/s). However, the Lp stimulated by submaximum effective doses of VP (1 and 10 μU/ml) was significantly lower in the IMCD of malnourished than normal rats. These findings indicate that the concentration defect observed in malnourished rats was due to a defect in NaCl transport in the thick ascending limb of Henle, and a partial resistance of IMCD to VP-stimulated water permeability.