Angiotensin II and its metabolites, [des-Asp-1]-angiotensin II (angiotensin III), [des-(Asp-1,Arg-2)]angiotensin II, and [des-(Asp-1,Arg-2,val-3)]angiotensin II, were measured in arterial and venous plasma from normal rats. Paper chromatography was used to separate the peptides which were measured with a radioimmunoassay method using an angiotensin II antiserum which had a 100% cross-reaction with each of these metabolites. A mean of 33% of the immunoreactive material in arterial plasma was angiotensin II, 58% angiotensin III, and 9% [des-(Asp-1,Arg-2)]angiotensin II/[des-(Asp-1,Arg-2,Val-3)]angiotensin II. A similar distribution was found in venous plasma. This and other evidence indicates that angiotensin-induced aldosterone secretion in the rat, unlike that in other mammalian species so far studied, may be mediated by angiotensin III.