The response to rapid infusion of 2.5% saline solution was examined in 21 patients 1–15 years after an episode of acute poststreptococcal glomerulonephritis. 12 were normotensive and 9 hypertensive; 11 had minimal proteinuria; renal biopsies showed varying degrees of glomerular sclerosis in 9 and were normal in the remainder; filtration rates ranged from 151 to 26 ml/min. Exaggerated natriuresis during the saline load occurred in 19 of the 21. The occurrence or magnitude of natriuresis was not dependent on the level of filtration rate or the presence of hypertension. The renal mechanism, which could not be attributed to increase in filtration rate, decrease in peritubular oncotic pressure, or increase in intrarenal pressure, is unexplained. It is suggested that this remarkably altered reactivity to saline loading reflects the presence of diseased nephrons and constitutes further evidence that irreversible renal damage occurs commonly following acute poststreptococcal glomerulonephritis.