Chlordimeform (CDM), a formamidine insecticide and monoamine oxidase (MAO) inhibitor, has recently been shown to produce profound changes in visual evoked potentials of hooded rats (Dyer and Boyes, The Toxicologist, 3: 13, 1983). Two experiments were performed to determine if the evoked potential changes were a result of MAO inhibition. In the first, the degree of MAO inhibition in the brains of rats treated with CDM (1.0 to 100 mg/kg i.p.) was compared with that produced by MAO inhibitor pargyline (0.3 to 30 mg/kg i.p.). Both compounds preferentially inhibited MAO-B, although MAO-A was substantially inhibited at higher dosages. Pargyline was a relatively more potent MAO inhibitor than CDM, but not more efficacious. In the second study, pattern reversal evoked potentials (PREPs) and flash evoked potentials (FEPs) were recorded from groups of rats after treatment with either saline, 0.4 mg/kg pargyline, 20 mg/kg pargyline, or 40 mg/kg CDM.