We have shown previously that angiotensin-(1-7) (Asp-Arg-Val-Tyr-Ile-His-Pro) is a biologically active endogenous angiotensin which is a major product of angiotensin I processing by an angiotensin converting enzyme (ACE)-independent pathway. Intense staining for angiotensin-(1-7) immunoreactivity was demonstrable in brain areas related to the maintenance of hydromineral balance, suggesting the involvement of this peptide in this process. In the present study we investigated the antidiuretic effect of angiotensin-(1-7) by determining its effect on the water diuresis produced by an ip water load (5 ml/100 g) in male Wistar rats. The peptide had a pronounced antidiuretic effect when administered peripherally in doses ranging from 5.5 to 22 pmol/100 g. In contrast, angiotensin II presented only a small effect with the highest dose used (20 pmol/100 g). Comparison of the potency of angiotensin-(1-7) and vasopressin (AVP) showed that both peptides act in the same molar range although AVP was slightly more potent than angiotensin-(1-7). Urine volumes for 22 pmol/100 g angiotensin-(1-7) were 0.85 +/- 0.26 and 3.47 +/- 0.36 ml for hours 1 and 2, respectively, whereas they were 0.54 +/- 0.40 and 2.38 +/- 0.64 ml for 10 pmol/100 g AVP. There was apparent additivity of effect when 10 pmol of each peptide were administered simultaneously (0.0 and 1.72 +/- 0.45 ml vs 2.58 +/- 0.45 and 3.85 +/- 0.35 ml for control for hours 1 and 2, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)