There is a widespread flurry of activity to uncover new compounds which will possess such monoamine-oxidase inhibitory effects with a minimum of toxicity. For these reasons we have devised this simple test, applicable to man, realising that further refinements in its application will be forthcoming and hoping that this type of procedure will place M.A.o.inhibition therapy on a rational basis. It will also be interesting to determine whether there are disease states in which M.A.O. activity is impaired.