Eight normal subjects (3 females and 5 males) were studied using intravenous L-[11C] deprenyl and positron emission tomography. In a single blind study one subject received tracer alone, one subject received an oral pre-dose of 20 mg of L-deprenyl and 6 subjects received oral pre-doses of 10 to 50 mg of a novel reversible MAO-B inhibitor (Ro 19-6327). Dynamic PET scans beginning 12 h after the oral dose were collected over 90 min and arterial blood was continuously sampled. Data analysis was modelled for two tissue compartments and using an iterative curve fitting technique the value of the rate constant for irreversible binding of L-[11C] deprenyl to MAO-B (k3) in whole brain was obtained for each subject. The dose response curves obtained indicated that a dose of at least 0.48 mg.kg-1 of Ro 19-6327 was necessary for greater than 90% decrease in whole brain k3. Inhibition of MAO-B in platelets isolated from blood samples taken at the time of scanning correlated strongly with decrease in whole brain k3 (r = 0.949). The results indicate that PET can be used to determine the dose of Ro 19-6327 necessary to inhibit greater than 90% of brain MAO-B. This technique is an attractive alternative to traditional large scale patient-based dose-finding studies. Moreover it is shown that inhibition of platelet MAO-B can be used as a marker for central MAO-B inhibition with Ro 19-6327.