1. The changes in apical Cl- permeability of Colony 29 human colonic epithelial monolayers were estimated from the rate constant of 125I- efflux from tissues loaded with the isotope. 2. Forskolin was used to increase intracellular concentrations of adenosine 3:5' cyclic-monophosphate (cyclic AMP), and A23187 to increase intracellular free Ca2+ (Cai). Both treatments increased the rate constant for 125I- efflux, indicating an increase in apical Cl- permeability. 3. Lysylbradykinin (LBK) also increased the rate constant for 125I- efflux, sometimes biphasically. Chelation of intracellular Ca2+ with BAPTA or prevention of prostaglandin formation with piroxicam, attenuated but did not eliminate the effect of LBK. It is concluded that LBK affects 125I- efflux through the agency of both cyclic AMP and Ca2+. 4. Ba2+ attenuated the effect of LBK and A23187 on 125I- efflux, but had no effect on the action of forskolin. It is concluded that Ca2+ has a major effect on K+ channels, the resulting hyperpolarization increasing the driving force for 125I- efflux. A secondary effect on Ca(2+)-sensitive Cl- channels is possible. By contrast, cyclic AMP exerts it major effect on apical Cl- channels. 5. Using a Cl- sensitive fluorescent dye, MEQ, the intracellular chloride concentration, Cli was estimated to be around 30 mM, which was increased to around 50 mM by forskolin, suggesting cyclic AMP could activate the Na-K-2Cl co-transporter. 6. MEQ fluorescence was used to estimate Cl- influx and efflux rates of epithelial cells. These were increased three fold by forskolin and dibutyryl cyclic AMP and two fold by LBK and histamine. 7. It is concluded that LBK increases electrogenic chloride secretion in Colony 29 epithelia through the generation of second messengers cyclic AMP and Ca2+, each of which may act on both apical and basolateral membranes.