To determine the effects of chronic maternal renal insufficiency on fetal renal function, 9 fetuses whose mothers underwent subtotal nephrectomy at least 2 months before mating (STNxF) and 7 fetuses from intact ewes (IntF) were studied (126-128 days gestation, term=150 days). STNxF had lower haematocrits (P<0.05), plasma chloride (P<0.01) and creatinine levels (P<0.01) and the length to width ratio of their kidneys was reduced (P<0.05). They excreted twice as much urine (P<0.05) and sodium (P<0.01). Total and proximal fractional sodium reabsorptions were lower in STNxF (P=0.01, P<0.05); distal delivery of sodium (P<0.05) and distal fractional sodium reabsorption were higher (P<0.05). They tended to have suppressed renin levels (P=0.06). Infusions of amino acids (alanine, glycine, proline and serine at 0.32 mmol/min for 1 h and 0.64 mmol/min for 2 h i.v.), known to stimulate renal blood flow and glomerular filtration rate in fetal sheep, did so in IntF (P<0.01). Arterial pressure also increased (P<0.01). These effects were not observed in STNxF. In summary, chronic maternal renal insufficiency was associated with profound alterations in fetal renal excretion of fluid and electrolytes and impaired renal haemodynamic and glomerular responses to amino acid infusion. Whether or not these marked changes in the renal function of fetuses carried by STNx ewes are associated with alterations in renal function in postnatal or adult life remains to be determined.