Tannin inhibits cAMP production in bovine AEC in part by inhibiting adrenergic receptor binding. The purpose of this study was to determine whether tannin affected other parts of the adrenergic-cAMP signal transduction pathway by examining the effect of tannin on GTP-regulatory (G) proteins. cAMP production in confluent AEC was measured in the presence of forskolin, a direct activator of adenylate cyclase, pertussis toxin, an inhibitor of Gi proteins and cholera toxin, an activator of Gs proteins. In cells pretreated with 50 g/ml tannin, forskolin-stimulated cAMP production was reduced to basal levels while pertussis toxin cAMP levels were unaffected. cAMP levels were diminished in cells pretreated with 50 g/ml tannin followed by cholera toxin stimulation. We conclude that tannin inhibits the production of cAMP in AEC by direct effects upon adenylate cyclase or by possibly altering G protein regulation.