1. Effects of 5-hydroxytryptamine (5-HT) on neuronal nicotinic acetylcholine (ACh) receptor channels were investigated by expressing cloned channel subunits in Xenopus oocytes. 2. When channels were expressed with a combination of alpha 3 and beta 4 subunits, 5-HT (10 to 300 microM) reversibly inhibited an inward current activated by 100 microM ACh in a concentration-dependent manner. The inhibition was also observed when alpha 3 subunit was combined with beta 2 subunit instead of beta 4 subunit, or beta 4 subunit was combined with alpha 2 or alpha 4-1 subunit instead of alpha 3 subunit to express channels. 3. Compounds known to antagonize at 5-HT receptors (LY53857, metoclopramide and propranolol) exhibited an agonistic effect: they inhibited the ACh-activated current. 4. The results suggest that 5-HT inhibits recombinant neuronal nicotinic receptor channels through a binding-site distinct from conventional 5-HT receptors. The binding-site may not be attributed to a unique type of channel subunits.