1. The S-K+ conductance was isolated by voltage-clamping near the resting potential pleural mechanosensory neurons of Aplysia in culture. This background conductance is modulated in opposite directions by two distinct, transmitter-controlled second-messenger cascades: it is enhanced by the peptide FMRFamide through the 12-lipoxygenase pathway of arachidonic acid, and it is decreased by serotonin (5-HT) through adenosine 3',5'-cyclic monophosphate (cAMP)-dependent phosphorylation. 2. The dose-dependent activating effect of FMRFamide (0.01-500 microM) on the S-K+ conductance was measured in the presence and the absence either of 1-100 microM 8-bromo-cAMP (8b-cAMP, a membrane-permeable and hydrolysis-resistant analogue of cAMP), or of 0.01-0.1 microM 5-HT. 3. When 8b-cAMP was applied, it produced a slow inward current response due to closure of the S-K+ conductance. This response was antagonized by FMRFamide in a dose-dependent mode. Application of 100 microM FMRFamide, in the presence of 1-10 microM 8b-cAMP, produced an outward current response larger than the control FMRFa response and equal to the sum of the responses to FMRFamide alone and to 8b-cAMP alone. Similarly, at 500 microM, FMRFamide completely antagonized the closing action of maximal 8b-cAMP levels (100 microM). This observation confirms previous work that indicated that FMRFamide can reopen S-K+ channels closed by FMRFamide. 4. In contrast, in the presence of moderate concentrations of 5-HT (0.01 microM), which produce a slow inward current due to the closing of the S-K+ conductance, FMRFamide elicited a response that only partially antagonized this 5-HT action. Under maximal 5-HT concentrations (0.1 microM), the 5-HT response was not antagonized by any FMRFamide concentration: instead, the FMRFamide response was smaller than the control response without 5-HT. This experiment suggests that 5-HT, with an action independent from cAMP, inhibits the effect of FMRFamide on the S-K+ channel. 5. The dose-dependent inhibitory effect of 5-HT (0.001-10 microM) on the S-K+ conductance was measured in the presence and the absence either of FMRFamide (1-50 microM), which stimulates the release and metabolism of arachidonic acid in Aplysia sensory neurons or of arachidonic acid (25 microM). 6. Under these conditions, supramaximal concentrations of 5-HT could not completely suppress the slow outward current evoked by FMRFamide or by arachidonic acid, indicating that a component of the arachidonic-mediated response to FMRFamide is resistant to actions that maximally increase the S-K+ channel phosphorylation.(ABSTRACT TRUNCATED AT 400 WORDS)