1. In the nephrotic syndrome the kidneys retain salt and water, which leads to oedema formation. The site of this sodium retention has been localized in the cortical collecting tubule by micropuncture studies. Whether or not this phenomenon is an intrinsic renal problem or is the consequence of changes in hormonal activities is still a matter of discussion. 2. Using the model of puromycin aminonucleoside-induced nephrotic syndrome in the rat, we measured Na+,K(+)-ATPase activity in nephron segments from control and nephrotic rats and investigated the regulatory role of aldosterone and endogenous-ouabain-displacing factor. 3. Nephrotic animals had a twofold increase in Na+,K(+)-ATPase activity in the cortical collecting tubule only (control versus nephrotic: 1065 +/- 68 versus 2081 +/- 274 pmol h-1 mm-1, P = 0.036), which was not modified by adrenalectomy and was independent of the kidney content of endogenous ouabain-displacing factor. Na+,K(+)-ATPase activity in the cortical collecting tubule correlated with the sodium balance in both control and nephrotic rats. 4. The data are consistent with the view that sodium retention in this model of the nephrotic syndrome is a primary event, i.e. an increase in sodium transport throughout the cortical collecting tubule expressed as a twofold increase in Na+,K(+)-ATPase activity which is no longer under hormonal regulation (aldosterone and endogenous ouabain-displacing factor).