Monitoring platelet monoamine oxidase activity in plasma of depressed patients receiving monoamine oxidase inhibitors is used as an indicator of therapeutic dosage and efficacy. In this study enzyme activity in platelets vs platelet rich plasma was compared. An aliquot of platelet rich plasma (PRP) was incubated with 4-hydroxy-3-methoxy-benzylamine as substrate. After stopping the enzymatic reaction, m-hydroxybenzaldehyde was added to the mixture as internal standard. The reaction mixture was extracted with toluene to isolate vanillin, the deamination product of the substrate, and the internal standard. The toluene layer was collected and back extracted into dilute tetramethylammonium hydroxide. An aliquot of the aqueous layer was chromatographed on a non-silica resin base reversed phase column with an alkaline phase. The peaks were detected by an absorbance detector at 350 nm. There was no significant difference in the decrease of monoamine oxidase activity when determined using platelet rich plasma versus washed platelets.