The biological activities of angiotensin II antagonists upon basal and angiotensin II-stimulated aldosterone production were evaluated in an isolated canine glomerulosa cell preparation. Our investigations found that the most potent competitive antagonist of angiotensin II-stimulated aldosterone production was the [Sar1, Ile8] derivative of angiotensin II. However, this peptide was also a partial agonist at concentrations required to inhibit the steroidogenic effect of angiotensin II on dog adrenal cells, and never reduced aldosterone production to basal levels.