These experiments examined whether renal growth and the fetal renin-angiotensin system could be stimulated by infusion of amino acids and whether chronic amino acid infusions restored glomerulotubular balance, which had been disrupted during 4-h infusions. Five fetal sheep aged 122 ± 1 days gestation received an infusion of alanine, glycine, proline and serine in 0.15 M saline at 0.22 mmol/min for 7 days. Six control fetuses were given saline at the same rate (5 ml/h). Kidney wet weights after amino acid infusion were 28% larger than control fetuses ( P < 0.05), and renal angiotensinogen mRNA levels were ∼2.6-fold higher ( P < 0.005). Circulating renin levels and renal renin mRNA levels were suppressed ( P < 0.05), and renal renin protein levels tended to be lower. Arterial pressure was increased, and there was a marked, sustained natriuresis and diuresis. Glomerular filtration rate and filtered sodium were ∼two-fold higher throughout infusion ( P < 0.05). Fractional proximal sodium reabsorption, suppressed at 4 h (from 73.4 ± 6.5 to 53.7 ± 10.2%), did not return to control levels (36.1 ± 3.4% on day 7, P < 0.05). Distal sodium reabsorption was markedly increased (from 79 ± 25 to 261 ± 75 μmol/min by day 7, P < 0.005), but this was not sufficient to restore glomerulotubular balance. The resultant high rates of sodium excretion led to hyponatremia and polyhydramnios. In conclusion, long-term amino acid infusions increased renal angiotensinogen gene expression, kidney weight, and distal nephron sodium reabsorptive capacity but failed to restore proximal and total glomerulotubular balance.