Moclobemide, an inhibitor of MAO-A, does not increase daytime plasma melatonin levels in normal humans.

1. Plasma melatonin concentrations were determined after administration of single oral doses (100, 200 and 300 mg) of moclobemide, a reversible inhibitor of monoamine oxidase (MAO) with predominant effects on the A-type of the enzyme, to eight young, healthy male volunteers in a double-blind, random-order, placebo-controlled study. The investigation was later continued in an open fashion by giving a single 10 mg dose of the MAO-B inhibitor deprenyl to the same subjects. 2. Neither drug had any effects on plasma melatonin levels, in spite of very marked MAO-A inhibition after moclobemide (as evidenced by up to 79% average decreases in the plasma concentrations of 3,4-dihydroxyphenylglycol, a deaminated metabolite of noradrenaline) and over 90% inhibition of MAO-B activity in blood platelets after deprenyl. 3. It is concluded that daytime human plasma melatonin levels do not accurately reflect MAO-A inhibition in acute drug studies.