Chlordimeform (CDMF) is a member of a relatively new class of insecticide/acaricide, the formamidines, which has rather unique effects. Unlike the widely used organophosphate and carbamate insecticides, CDMF does not appear to inhibit cholinesterases, and intoxication is not accompanied by signs of cholinergic over-stimulation. On the contrary, the available evidence suggests that one of the many actions of CDMF may be on monoamine-containing neurons in the central nervous system. CDMF in rats has been shown in vitro to inhibit hepatic monoamine oxidase and in vivo to increase whole-brain levels of 5-hydroxytryptamine (5-HT) and norepinephrine. In addition, some behavioral effects seen after large dosages have been reported to resemble those obtained after administration of agents which effect neuronal release of 5-hydroxytryptamine, either p-chloroamphetamine alone or 1-tryptophan in combination with a monoamine oxidase inhibitor.