Formetanate (FMT) is a formamidine acaricide/insecticide with a carbamate moiety in its molecular structure. FMT-induced lethality is reportedly due to inhibition of acetylcholinesterase. Here evidence is reported of the neurochemical basis for the sublethal, behavioral effects of FMT in rats. Injections of scopolamine and methylscopolamine 15 min before FMT blocked the response rate suppression, whereas pretreatment with either mecamylamine or hexamethonium did not. These data suggest that FMT acts as an indirect agonist on central and peripheral muscarinic receptors, by inhibiting acetylcholinesterase, to produce changes in schedule-controlled responding.