Background. We previously demonstrated that vasopressin (AVP) produces a sustained increase in Na+ reabsorption by the isolated perfused cortical collecting duct (CCD) from rats on a normal diet, and that this effect is synergistic with that of pharmacological doses of deoxycortico-sterone (DOG) or physiological levels of aldosterone. The present experiments examined the effect of AVP under the more physiological circumstances when plasma aldosterone was elevated by prior volume depletion. Methods. Rats were volume depleted by a single dose of furosemide followed by a low-salt diet (0.3% NaCl) for four to nine days. Some of these rats were also implanted with a pellet containing 2.5 mg DOC. Rats in a third group were not injected with furosemide but were implanted with the DOC pellet and maintained on a standard (~1% NaCl) diet. CCD were perfused and the lumen-to-bath Na+ flux (J(Na)), transepithelial voltage (V(T)), and osmotic water permeability (P(f)) were measured in the presence and absence of 200 pM AVP. Results. Although Na+ depletion by a single injection of furosemide and the low salt diet elevated plasma aldosterone and V(T), J(Na) remained low and there was a decreased response to AVP in comparison with DOC-treated rats on a standard diet. In CCD from rats on the low salt-diet with DOC, J(Na) was less than observed in CCD from DOC-treated rats on a standard diet. AVP-dependent P(f) in CCD from rats on the low-salt diet, with or without DOC treatment, was also markedly lower. Conclusions. We interpret the results to demonstrate that maximal rates of Na+ reabsorption in the CCD depend not only on the synergistic stimulatory effects of aldosterone and AVP, but also require normal to high rates of salt delivery in vivo for the effects of the hormones on Na+transport to be maximized in vitro.