We have studied the effects of platelet-activating factor (alkyl-acetyl-GPC) on conversion of angiotensin I to II. Platelet-activating factor (PAF) had a novel effect on angiotensin I conversion. Pulmonary artery endothelial cells converted 30% of [125I]angiotensin I to angiotensin II in the absence of PAF, but their activity was greatly stimulated in its presence. When PAF was added to pulmonary artery endothelial cells, the conversion of angiotensin I to II was enhanced up to 68%. Maximal stimulation without cellular damage was achieved at 10(-5) M PAF. LysoPAF did not stimulate the conversion of angiotensin I to angiotensin II at any concentration used.