This study was designed to see whether or not increases in monoamine oxidase (MAO) specific activity that follow chronic treatment of rats with L-dihydroxyphenylalanine (L-DOPA) could be modified by benserazide (Ro 4-4602), an inhibitor of L-aromatic amino acid decarboxylase, and to compare the properties of the increased MAO activity with those of control animals. 2. Male wistar rats were treated with L-DOPA (250 mg/kg) and benserazide (40 mg/kg) either alone or in combination for 10 days. 3. The activity of MAO in homogenates of heart, kidney, liver and brain was measured with 5-hydroxytryptamine (5-HT) and benzylamine (BZ). 4. The significant increases in MAO specific activity seen in heart and kidney following L-DOPA treatment could be reduced or prevented by benserazide. 5. Use of the selective MAO inhibitor clorgyline showed that the increases in MAO specific activity, when measured with either 5-HT or BZ were due to an increase in the number of active centres of MAO-A, in the rat heart. 6. There was a significant increase in the Vmax of the enzyme reaction with 5-HT, in the rat heart and kidney homogenates. 7. It is concluded that L-DOPA increases the specific activity of MAO-A in rat heart and kidney as a result of its decarboxylation.
