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The effects of some organophosphate pesticides, e.g. lebaycid, metacid and metasystox on the monoamine oxidase (MAO) activity in rat brain mitochondria have been studied. These pesticides cause significant inhibition of MAO activity in vitro but have negligible effects on its activity in vivo.

Chloropromazine (CPZ) and imipramine at a concentration of 1 x 10(-3) M inhibit rat brain mitochondrial monoamine oxidase activity in vitro by 70 and 55% respectively, while lithium, even at a concentration of 0.05 M, inhibits the activity of this enzyme very negligibly (4%). In vivo, these drugs at a dose level of 56 mg CPZ, 76 mg imipramine and 76 mg lithium chloride/Kg body wt., did not cause any observable variation from normal in brain mitochondrial monoamine oxidase activity.