A new method is described for the evaluation in vivo of histidine decarboxylase inhibitors. Urinary methylhistamine excretion is approximately trebled in rats treated simultaneously with the histaminase inhibitor aminoguanidine and the monoamine oxidase inhibitor pargyline, thus providing a high base-line against which inhibiton of methylhistamine formation, and hence of histamine formation, can be measured. Variations in the recovery of methylhistamine are compensated for by addition of a standard amount of radioactive methyl-histamine to each urine sample. The method has the advantage over previously described methods of requiring less radioactive material and fewer animals which, moreover, do not have to be killed and so can be re-used. In addition, the time course of inhibition can be assessed.