Isolated glomeruli from normal rats were incubated with platelet-activating factor 10(-6) M at variable incubation times (8, 15, 30 and 45 min) and with different concentrations (0 to 10(-6) M) for 20 min. In addition, the platelet-activating factor effect (10(-6) or 10(-8) M, 20 min) was tested in the presence of BN-52021 (10 or 50 micrograms/ml), verapamil (10(-5) M), acetylsalicylate of lysine (10(-3) M), and in a free-calcium media with EGTA 2 mM. Glomerular microphotographs were taken before and 20 min after adding the substances, and glomerular cross-sectional area was measured using a computerised technique. Platelet-activating factor induced a significant time-dependent reduction in glomerular cross-sectional area, from a concentration of 10(-8) M. BN-52021, verapamil, and the free-calcium media inhibited platelet-activating factor-induced reduction of glomerular cross-sectional area, but acetylsalicylate of lysine did not. Platelet-activating factor-induced reduction in glomerular cross-sectional area seems to be dependent on the interaction of platelet-activating factor with a specific glomerular receptor, with a subsequent modification of the intracellular concentration of calcium. Arachidonic acid metabolites from the cyclo-oxygenase pathway do not seem to be involved in these phenomena. Results suggest that platelet-activating factor could modulate glomerular filtration rate not only by inducing changes in systemic or intrarenal haemodynamics, but also by modifying the filtration surface, thus reducing the Kf.