Guanylin and uroguanylin are novel peptides that activate membrane guanylate cyclases found in the kidney and intestine. We compared the effects of these peptides in the isolated perfused rat kidney. Both peptides are natriuretic and kaliuretic in this preparation. Uroguanylin (0.19–1.9 μM) increased glomerular filtration rate from 0.77 ± 0.07 to 1.34 ± 0.3 ml ⋅ g⁻¹ ⋅ min⁻¹at the highest concentration. A maximal increase in Na⁺ excretion was achieved at 0.66 μM uroguanylin, with a reduction in fractional Na⁺ reabsorption from 78.7 ± 1.7 to 58.8 ± 4.4%. The highest dose of uroguanylin increased kaliuresis by 50%. Osmolar clearance doubled at the highest concentration of uroguanylin tested (P< 0.05). Guanylin also elicited a natriuresis and kaliuresis but appeared to be less potent than uroguanylin. The highest concentration of guanylin (1.3 μM) decreased fractional Na⁺ reabsorption from 73.9 ± 2.4 to 64.5 ± 4.0%, but lower doses were ineffective. Guanylin stimulated urine K⁺ excretion at the lowest concentration tested (0.33 μM) without any effect on Na⁺ excretion. These peptides may influence salt and water homeostasis by biological effects in the kidney that are mediated by the intracellular second messenger, cGMP.