We investigated the relationship between receptor-mediated increases in cytoplasmic Ca2+ concentration ([Ca2+]i) and increased microvessel permeability. In individually perfused venular microvessels of frog mesentery exposed to 10 microM ATP, [Ca2+]i increased from 59 +/- 7 to 172 +/- 21 nM within 1 min and then fell back toward control values. The corresponding peak increase in the hydraulic conductivity (Lp) of the microvessel wall was 5.7 +/- 0.5-fold relative to control. After removal of extracellular Ca2+, there was no significant increase in Lp, and the initial increase in [Ca2+]i was attenuated but not abolished. Depolarization of the endothelial cell membrane with high-K+ Ringer solution reduced the peak increase in [Ca2+]i to 106 +/- 7 nM and attenuated the increase in Lp 1.8 +/- 0.4-fold. The results conform to the hypothesis that Ca2+ entry into endothelial cells is required for acute increase in venular microvessel permeability by inflammatory agents and that the pathway for Ca2+ entry has the properties of a passive conductance pathway. Similar conclusions were reached in previous experiments in frog microvessels exposed to Ca2+ ionophores and perfusates with no plasma proteins. In venular microvessels of hamster mesentery exposed to ATP and bradykinin, a similar pathway for Ca2+ entry was demonstrated in the present experiments. We did not measure permeability changes in hamster microvessels in this study, but these microvessels respond to histamine and ionophores with a transient increase in permeability to macromolecules similar to that measured in frog microvessels [Am. J. Physiol. 268 (Heart Circ. Physiol. 37): H1982-H1991, 1995].