Intravenous infusion of platelet-activating factor (PAF or AGEPC) induces extensive acute hemorrhagic damage in the ra gastric mucosa. The effects of PAF have now been investigated on several microcirculatory parameters in the rat gastric mucosa. Infusion of PAF (25-100 ng X kg-1 X min-1 iv) dose-dependently reduced systemic arterial blood pressure and mucosal blood flow, as determined by hydrogen gas clearance. Microscopic observation likewise indicated a dose-dependent slowing of mucosal capillary blood flow, as determined by red blood cell velocity after PAF infusion with stasis of flow at the highest dose. These actions were not the consequence of vasoconstriction, since there was no significant change in arteriolar and venular vessel diameter in the submucosa during PAF infusion, although a dose-related slowing and stasis of blood flow was observed. There was no capillary leakage of plasma protein in the gastric mucosa during PAF infusion, as determined by a fluorescence marker (FITC-BSA) technique. The precursor and breakdown product, lyso-PAF (200 ng X kg-1 X min-1) had no significant action on any of these microcirculatory parameters. These observations support the suggestion that microvascular changes leading to stasis may contribute to the ulcerogenic actions of PAF.