The role of endothelium in the vasodilation of third order arterioles of cremaster muscle to a variety of vasoactive agents was investigated in pentobarbital-anesthetized rats. Changes in diameter to topical administration of agents were measured with image shearing, before and after mercury light/sodium fluorescein (light/dye) treatment of a 50- to 100-microns segment of the arteriole under study and were recorded with video microscopy. Before light/dye treatment, arachidonic acid (10(-5) M), prostaglandin E2 (5 x 10(-6) M), A23187 (2 x 10(-6) M), acetylcholine (10(-5) M), and adenosine (10(-4) M) elicited dilation between 75 and 106% of basal diameter. After light/dye treatment, dilations to arachidonic acid, A23187, and acetylcholine were completely eliminated; however, the responses to prostaglandin E2 and adenosine were not altered. These results indicate that light/dye treatment interferes with the production of or response to prostaglandins as well as other endothelial mediators, like endothelium-derived relaxing factor (EDRF). In a second series of experiments, bradykinin, in concentrations of 10(-9), 10(-8), and 10(-7) M elicited dose-dependent dilations, which were partially inhibited by indomethacin and completely abolished after additional light/dye treatment. Dilation of arterioles to adenosine was maintained throughout these experiments. These data suggest that vasodilation to bradykinin is mediated partly via prostaglandin production and partly via other endothelium-derived factor(s).