Previous studies have shown that angiotensin I (AI) is converted to angiotensin II (AII) by converting enzyme (CE) or by tonin. Since all work on tonin has been done in the rodent, its relevance to the primate is yet to be shown. This study examines the conversion of AI to AII by the cheek pouch microcirculation of the monkey (macaca fascicularis). In 9 of the 11 monkeys studied, the conversion of AI to AII was blocked by converting enzyme inhibitor (CEI). However, in 2 older monkeys, AI conversion was not altered by CEI although the vascular response to AI was attenuated by both the AII antagonist (saralasin) and the antibody to AII (Ab-AII). In these 2 older animals, renin substrate caused vasoconstriction that was not altered by CEI, but was reduced by saralasin and Ab-AII. It is concluded that the tonin-angiotensin system is not active in the young macaque, but may be present in the older animals.