Relationships between the electrical resistance (Rm) of cultured endothelium and the resistivity (rho) of the bathing solution have been investigated, using eight monolayers of bovine arterial endothelium. Bathing solutions of differing rho were prepared by mixing culture medium and isosmolar (290 mM) sucrose in various proportions. Relationships between 1/Rm and 1/rho were linear, as predicted, with a mean 1/Rm intercept of zero. The slopes of the relationships give a mean fractional area per unit path length (Ap/delta chi) of 12.7 cm-1 for the paracellular pathways of the cultured endothelium. This resembles previous estimates of Ap/delta chi for mammalian endothelium in arteries. The 1/Rm intercept of zero indicates that the major electrical conducting pathways of cultured arterial endothelium are paracellular.