The renal effects of urodilatin and atrial natriuretic peptide (ANP) were examined in conscious dogs during acute volume expansion maintained through independent infusions of water and NaCl. Peptide was infused in a step-up fashion, in 40-min periods at rates of 2.5, 12.5 and 50.0 ng kg-1 min-1 (ANP, urodilatin) or 0.5, 2.5 and 10.0 ng kg-1 min-1 (second series of urodilatin). ANP immunoreactivity (ANPir) in plasma was measured with an antibody showing 100% cross-reactivity with urodilatin. At 50 ng kg-1 min-1, ANP increased plasma ANPir some 10-fold (64 +/- 6 pg ml-1 to 728 +/- 82 pg ml-1) while urodilatin elicited a 25-fold increase in ANPir (78 +/- 13 pg ml-1 to 1645 +/- 204 pg ml-1). ANP doubled sodium excretion (61 +/- 12 to 124 +/- 30 mumol min-1, P < 0.05) while urodilatin increased sodium excretion to 269 +/- 45 mumol min-1. Both peptides approximately doubled urine flow. Urodilatin at 10 ng kg-1 min-1 increased sodium excretion from 57 +/- 17 mumol min-1 to 106 +/- 25 mumol min-1, i.e. similar to the response ANP elicited at 50 ng kg-1 min-1. In the time control series a significant fall in sodium excretion was observed, despite the continuous replacement of sodium and water. The results demonstrate that in identical, relatively high doses, the changes in sodium excretion, diuresis, heart rate (HR) and arterial pressures elicited by urodilatin are more pronounced than those of ANP.(ABSTRACT TRUNCATED AT 250 WORDS)