The action of 5-hydroxytryptamine (5-HT) on fetal (gamma-AChR) and adult (epsilon-AChR) muscle acetylcholine receptors was studied in transfected BOSC 23 cells expressing mouse AChRs and in acutely dissociated mouse muscle fibres. In transfected cells, coapplication of 5-HT (0.01-10 mM) with ACh reversibly reduced the amplitude and accelerated the decay of the ACh-activated whole-cell currents, in a dose- and voltage-dependent manner. 5-HT blocked faster and with higher potency the gamma-AChR than the epsilon-AChR. Cell-attached recordings from transfected BOSC 23 cells and embryonic or neonatal muscle fibres were made. When 5-HT (5 microM) was included in the patch pipette, ACh-evoked unitary events acquired a bursting behaviour, which was more pronounced for gamma- than epsilon-AChR, though it was enhanced by membrane hyperpolarization for both AChR species. There was no effect on the single-channel conductance. It is concluded that 5-HT behaves as an open-channel blocker of muscle AChRs, with higher efficacy on gamma- than on epsilon-AChR.