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The effects of atriopeptin III (API 11) on systemic haemodynamics were examined in 12 anaesthetized rats. Five minutes following intravenous injection (i.v.) of 10 n-g/kg APIII, cardiac output CO, measured by electromagnetic flowmetry, stroke volume and mean arterial pressure (MAP) decreased by 14, 13 and 8% (P<0.05), respectively, and total peripheral resistance (TPR) increased by 10% (P<0.05). Heart rate (HR) and left ventricular end-diastolic pressure (LVEDP) did not change. In order to examine cardiac performance, whole blood was infused into three groups of 12 rats each receiving either no injection, APIII (10 (xg/kg i.v.) or APIII (10 |xg/kg i.v.) plus a continuous infusion of phenylephrine to increase MAP to pre-injection levels. Cardiac performance curves did not differ among the three groups. These data indicate that the immediate decreases in MAP and CO produced in rats by a maximum natriuretic bolus dose of APIII were not mediated by a negative myocardial inotropic effect.