Secretion of a KCl-rich saliva in blowfly salivary glands is stimulated by the hormone serotonin (5-HT). The high folded apical membrane of the secretory cells contains a vacuolar-type proton pump (V-ATPase). V-ATPase energizes the apical membrane, thus fuelling a parallel nH⁺/K⁺ exchanger. 5-HT stimulates V-ATPase activity by increasing [cAMP]_i. cAMP may act on V-ATPase either via PKA (protein kinase A) or EPAC (exchange protein directly activated by cAMP). To distinguish between these possibilities, we determined the effect of PKA-selective or EPAC-selective cAMP-analogues on V-ATPase mediated H⁺-transport into the gland lumen. Microfluorometric measurements of luminal pH in blowfly salivary glands showed that superfusion of isolated glands with PKA-selective cAMP-analogues induces luminal acidification. Combining two PKA-activators, one of them with affinity for binding site A and the other one with affinity for binding site B of PKA, leads to a synergistic effect on V-ATPase acivity. Specific blockers of PKA inhibit the 5-HT induced luminal acidification. In contrast, activators of EPAC do not induce luminal acidification, but lead to slight alkalinization of the gland lumen. These findings suggest that PKA activates V-ATPase, whereas EPAC might have some inhibitory effect on the pump.