We studied the effects of mid-term enalapril administration on protein-load-induced renal responses in 10 patients with early chronic renal failure (serum creatinine 2.70 ± 1.0 mg/dl). The oral protein load was performed twice, before and after a 10-day therapy with enalapril. Glomerular filtration rate (125I-iothalamate clearance) rose from 22.5 ± 10.6 to 60.1 ± 32.8 ml/min after the protein load before enalapril; it did not change after the protein load during enalapril therapy. Percent fractional excretion of sodium, urinary osmolality and free water clearance were significantly affected only by the protein load before enalapril. Enalapril blunts the protein-load-induced changes in glomerular filtration rate and in tubular function; these effects might be mediated by angiotensin II blocka