Although the platelet-activating factor (PAF) is the most active inflammatory mediator known to date, little is known about its effects on the vascular endothelium and about the cellular and subcellular distribution of its receptor, already identified as a membrane protein of approximately 39 kDa. To better understand its functions we decided: i) to study PAF effects on a model microvascular bed (the rat cremaster), ii) to raise monoclonal antibodies against synthetic peptides reproducing short segments (14 and 16 amino acids) at the N and C terminal parts of PAF-receptor (PAF-R), iii) to determine the distribution of PAF-R on a number of microvascular beds. Topical application of the PAF on the cremaster led promptly to: i) opening of the venular and capillary endothelial junctions; ii) fenestration of the endothelium and iii) swelling, clustering and fusion of endothelial plasmalemmal vesicles. With the anti-N terminal antibody, we localized PAF-R by immunofluorescence on semithin frozen sections of lung, heart, diaphragm, kidney, and brain specimens. With the exception of brain, the signal was restricted primarily to the vascular endothelium. Using immunogold procedures, we localized the PAF-R in small clusters on endothelial surfaces and found it associated preferentially with the plasmalemma proper, rather than to any differentiated microdomain. A morphometric analysis revealed a greater signal density at the level of the venular endothelium than at the level of the endothelium of any other segment of the microvasculature. With the same antibody, we immunoprecipitated PAF-R from whole homogenates of the same tissues. The results obtained were in general agreement with the immunofluorescence tests.