MDL 72974A, a new irreversible selective inhibitor of monoamine oxidase (MAO)-B which is not metabolized to amphetamine-like compounds, is currently being developed for the treatment of Parkinson's disease. In this double blind, placebo controlled randomized study 24 healthy volunteers (n = 6/dose) received single oral doses of placebo, 1, 12 or 24 mg of MDL 72974A qd over two weeks. Sensitivity to orally administered tyramine was determined under fasting conditions before and after drug administration and the doses of tyramine yielding a 30 mmHg increase of SBP (PD30) compared. The 2-fold increase of tyramine sensitivity at end of treatment seen at all MDL 72974A dose levels, however, is within the variability range of the tyramine pressor response. MDL 72974A selectively inhibits MAO-B at doses up to 24 mg orally and has a favourable safety profile.