The 1-(1-phenylcyclohexyl)-2,3,4,5-tetrahydropyridinium metabolite derived from the psychotomimetic agent phencyclidine [1-(1-phenylcyclohexyl)piperidine] is converted to 1-(phenylcyclohexyl)-1,2,3,4-tetrahydropyridine-5-carboxaldehyde in the presence of rat brain and liver mitochondria. The C-5 formylation of synthetic 1-(1-phenylcyclohexyl)-2,3,4,5-tetrahydropyridinium perchlorate by N5-formyltetrahydrofolic acid (folinic acid) suggests that this biotransformation is mediated by a transformylation process involving this N5- or the corresponding N10-formyltetrahydrofolic acid.