In order to clarify mechanism of the formation of cinnamaldehyde (CNA) in incubation mixtures of tranylcypromine (TCP) with rat liver microsomes, the CNA formed under various incubation conditions were analyzed. For the purpose, HPLC method of the analysis of CNA was developed. The formation of CNA was found to be dependent on the incubation time and the amounts of microsomes added. In addition, exclusion of NADPH or NADP of NADPH-generating system in incubation mixtures resulted in the formation of markedly decreased amounts of CNA to 8.5 and 2.4%, respectively, relative to the amounts formed each in a standard system. The small amounts measured were comparable to those formed by incubation without microsomes or with boiled microsomes. The results clearly suggested that CNA is a metabolic product of TCP by rat liver microsomes though further studies are needed to suggest details of the steps to the formation of CNA from TCP and of the enzymatic entities involved in the formation of CNA.