Cultured central neurones from the American cockroach, Periplaneta arnericana, have been used to investigate the uptake of [3H]serotonin. The neurones accumulate [3H]serotonin from the extracellular medium by both a highand a low-affinity system. The activity of the high-affinity mechanism i s decreased by low temperature and metabolic poisons, and i s dependent on sodium and chloride ions. Both depolarising levels of external potassium ions and veratridine decrease the high-affinity uptake system, suggesting it is influenced by the transmembrane potential. The pyrethroid insecticides, deltamethrin and permethrin, enhance the inhibitory effect of veratridine. Pyrethroid enhancement is completely blocked by tetrodotoxin, and neither pyrethroid affects the uptake system in the absence of veratridine. Avermectin BI A is a powerful inhibitor of the high-affinity uptake system, and its effect i s blocked by picrotoxin. High-affinity uptake of [3H]serotonin i s inhibited by imipramine and amitriptyline; desipramine has no significant effect on this uptake. The activity of the high-affinity system is also reduced by 8-hydroxy-dipropylaminotetralin, a-methyl-serotonin, and l-(3-chloro-pheny1)piperazine. Dopamine, noradrenaline, octopamine, and the formamidine insecticides, chlordimeform and demethylchlordimerform, are moderate inhibitors of the high-affinity uptake system. The formamidine effect is not blocked by tetrodotoxin or picrotoxin.