In the absence of a monoamine oxidase inhibitor, the bulk of intravenously injected radioactively labelled beta-phenylethylamine was oxidized to phenylacetic acid. In the presence of pargyline, most of the label in tissues remained as unchanged phenylethylamine; small amounts of labelled phenylethanolamine, tyramine and octopamine were also identified. After intravenous injection of [14-C]phenylalanine, only very small amounts of [14-C] phenylethylamine could be located in urine and faeces. Beta-Phenylethylamine became concentrated in all tissues, including brain, following intravenous introduction both in the presence and absence of pargyline. Its clearance from these tissues and from brain regions was very fast.