Our previous study of the choroid plexus showed that elevation of cellular cyclic AMP stimulated HCO3 secretion into the cerebrospinal fluid (c.s.f.). To explore the mechanism of this stimulation, we have used micro-electrodes to observe the effect of phosphodiesterase inhibitors on membrane potentials, resistances and electromotive forces (e.m.f.) in the bull-frog plexus. The potential profile under control conditions was -45 mV across the apical membrane (Vvc) and -9 mV across the epithelium (Vvs) with respect to the c.s.f. Two-dimensional cable analysis indicated that 70% of adjacent epithelial cells were electrically uncoupled, and that the coupling coefficient for the remaining cells was only 0.12. The results of circuit analysis gave values for membrane resistance and e.m.f. across the apical membrane of 200 omega cm2 and -135 mV, across the basolateral membrane of 386 omega cm2 and -138 mV, and across the paracellular shunt of 20 omega cm2 (the shunt e.m.f. was assumed to be negligible). Addition of phosphodiesterase inhibitors (10 mM-theophylline or 1 mM-3-isobutyl-1-methylxanthine (IBMX) ) depolarized Vvc by 18 mV and Vvs by 2 mV. Circuit analysis showed that the inhibitor reduced the apical membrane resistance by more than 60% and depolarized the apical e.m.f. by 70 mV without affecting the basolateral membrane or the shunt parameters. In Cl-free solutions IBMX depolarized Vvc and decreased the apical membrane resistance similar to that observed in regular buffer solution. However, in HCO3-free buffer solutions, the effects of IBMX on Vvc and membrane resistances were insignificant. In thirty-five cells in sixteen tissues a linear relationship was observed between the magnitude of the spontaneous membrane potential (Vvc) and the magnitude of IBMX-induced depolarization (delta VIBMXvc): delta VIBMXvc = -0.8 Vvc -15 (in mV). This suggests that HCO3 is accumulated within the epithelium above electrochemical equilibrium. We conclude that cyclic AMP increases HCO3 secretion across the choroid plexus by increasing the apical membrane HCO3 conductance.