The influence of hypericin, hypericum total extract, and hypericum fractions on the activity of MAO and COMT, prepared in vitro from pork liver, were investigated in several concentration steps. An inhibition of MAO could be shown in the following concentrations (extract correlated to a mean molecular value of 500): hypericin to 10⁻³ mol/ L, hypericum total extract to 10⁻⁴ mol/L, one extract ... [Show full abstract] fraction up to 10⁻⁵. A COMT inhibition could not be shown for hypericin, with hypericum extract to 10⁻⁴ mol/L and with two extract fractions also up to 10⁻⁴ mol/L. The MAO inhibiting fraction contained hypericins as well as flavonols, the COMT-inhibition fraction being mainly flavonols and xanthones. The concentrations of inhibition shown might not be sufficient to explain the clinically proven antidepressive effect of hypericum particularly with regard to the inhibition of MAO activity.