AbstractN, N'‐Dimethyl 1‐chloro‐2‐naphthaleneethanimidamide monohydrochloride is a potential psychotherapeutic agent for which a sample of carbon‐13 labelled material was required for metabolism studies. A sample of 90% enriched N, N'‐dimethyl 1‐chloro‐2‐naphthaleneethanimidamide monohydrochloride was prepared from 90% carbon‐13 enriched potassium cyanide and 2‐methylnaphthalene in five steps. Chlorination of 2‐methylnaphthalene with sulfuryl chloride gave 1‐chloro‐2‐methylnaphthalene in 69% yield. Bromination of 1‐chloro‐2‐methylnaphthalene with N‐bromosuccinimide in carbon tetrachloride in the presence of catalytic benzoyl peroxide afforded 2‐(bromomethyl)‐1‐chloronaphthalene in 59% yield. Treatment of 2‐(bromomethyl)‐1‐chloronaphthalene with potassium cyanide in aqueous ethanol afforded13C‐1‐chloro‐2‐naphthylacetonitrile in 89% yield. The nitrile was converted to the imidate ester hydrochloride in 89% yield under Pinner reaction conditions (1). This intermediate was treated with excess methylamine in ethanol at 40‐55°C to give the title compound in an overall yield of 27% from 2‐methylnaphthalene. The effects of the13C labelling on the infrared and1H and13C NMR sp