To test the effect of exogenous atrial natriuretic factor (ANF) on the kidney function of rats with chronic renal insufficiency, we examined the responses to bolus infusions of atriopeptin III in two models of renal insufficiency, namely, nephrosclerosis and glomerulonephritis. Hypertensive Dahl S rats receiving a high (8%) NaCl diet for 12 weeks, and rats with anti-glomerular basement membrane nephritis, both of which had glomerular filtration rate (GFR) and effective renal plasma flow (ERPF) less than half those of respective control rats, exhibited similar relative increases In GFR and ERPF in response to atriopeptin III as did controls. Natriuresis, diuresis, and decreased blood pressure were as prominent in rats with either form of renal disease as in control rats. Rats receiving high NaCl intake, irrespective of renal function, had relatively exaggerated responses compared with rats receiving a normal NaCl diet. We conclude that rats with chronic renal insufficiency of diverse causes respond briskly to ANF despite presumed hyperperfused and hyperfiltering nephrons. Thus, diseased kidneys exhibit substantial renal functional "reserve." The renal response to ANF appears to be influenced by NaCl intake in both normal animals and those with renal insufficiency.