AbstractLorcainide monohydrochloride, N‐(4‐chlorophenyl)‐N‐/1‐(2‐methylethyl)‐4‐piperidinyl/benzeneacetamide monohydrochioride, is a new orally active antiarrhythmic drug.Incorporation of tritium was achieved by reduction of 4‐chloro‐N‐/1‐(1‐methylethyl)‐4‐piperidinylidene/benzenamine with tritiated sodium borohydride to N‐(4‐chlorophenyl‐1‐1‐methylethyl)‐4‐piperidinamine. This compound was converted in situ into the amide derivative.The radioactive yield of the last two synthesis steps was 82.7 % spread over two fractions with specific activities of 1.1 Ci/mmol and 0.046 Ci/mmol. The labelled compounds were radiochemically pure according to thin‐layer chromatography in three solvent systems, and high‐performance liquid chromatography. The radiochemical stability of lorcainide monohydrochloride investigated in both acidic and alkaline media for 1 hour at 60°C was found to be excellent.