The effect of human urodilatin (hURO) on kidney function has been examined in stable recirculating isolated perfused rat kidney. The concentration-dependent effects of urodilatin (URO) on isolated perfused rat kidney were analyzed at different perfusion pressures. The experiments show that at perfusion pressures of 100 or 130 mm Hg, 100 nmol/l hURO cause a significant increase of urine flow and a significant decrease of the fractional sodium reabsorption from 95.2 (SD 0.9, n = 5) to 85.6% (SD 0.9, n = 5) and 86.8 (SD 1.6, n = 5) to 77.6% (SD 3.2, n = 5), respectively. In contrast, at a perfusion pressure of 80 mm Hg sodium excretion was not increased by URO. The glomerular filtration rate was unchanged. Furthermore, the renal effects of hURO were compared with rat urodilatin (rURO) and the circulating form of rat cardiodilatin/atrial natriuretic peptide (rCDD/ANP 99-126). hURO, rURO, and rCDD/ANP produced a concentration-dependent increase in sodium excretion. However, differences in natriuretic efficiency are observed. rCDD/ANP causes a decrease in fractional sodium reabsorption from 93.6 (SD 2.6, n = 5) to 86.8% (SD 2.4, n = 5), similar to hURO. rURO shows a significantly higher natriuretic effect decreasing fractional sodium reabsorption from 92.0 (SD 0.7, n = 5) to 79.1% (SD 1.2, n = 5). From these results we conclude that the response of the isolated kidney to URO is critically dependent on the perfusion pressure and that URO exhibits tubular action on renal sodium excretion. The observed differences among the tested peptides on renal function may be due to species differences in the peptide sequence.