1. The catecholamine (CA) and 5-hydroxytryptamine (5-HT) content of the rat's brain has been examined after treatment with various drugs. The mean values for untreated animals were 0.43 μg./g. wet tissue for CA and 0.46 μg./g. for 5-HT. 2. By treatment with 100 mg./kg. of iproniazid intraperitoneally 20 hours before killing the animals, the CA content increased by about 65% and the 5-HT content by about 200%. 3. This increase of CA and 5-HT in the rat's brain could be almost completely antagonized by an intraperitoneal injection of 20 mg./kg. of chlorpromazine. 4. The in vivo action upon monoamine oxidase of iproniazid and chlorpromazine has also been examined. Chlorpromazine did not influence the activity of the monoamine oxidase in the rat's brain nor did it interfere with the inhibitory action of iproniazid upon this enzyme. 5. Reserpine (5 mg./kg. intraperitoneally) caused a depletion of the brain amines. This depletion was antagonized by pretreatment of the animals with iproniazid. The iproniazid-induced inhibition of the amine depleting effect of reserpine could also be abolished by chlorpromazine. A potentiation of the amine depleting action of reserpine by chlorpromazine could not be found in our experiments. 6. The biological significance of these findings has been discussed.