The effects of coptisine on monoamine oxidase (EC 1.4.3.4; MAO) activity in mouse brain were investigated. Coptisine showed an inhibitory effect on MAO activity with a concentration-dependent manner. Coptisine exhibited 51.0% inhibition of MAO activity at $9;{mu}M$. The $IC_{50}$ value of coptisine was $8.7;{mu}M$. Coptisine inhibited MAO activity competitively with kynuramine as a substrate. The $K_i$ value of coptisine was $4.1;{mu}M$. These results indicate that coptisine functions to regulate the catecholamine content at biologically active sites.