AbstractThree syntheses of radiolabelled 2‐[2‐(2‐N,N‐Dimethylaminomethyl‐5‐furanyl‐methylthio)ethylamino]‐5‐(6‐hydroxy‐4‐picolyl)‐4‐pyrimidone trihydrochloride (donetidine trihydrochloride) are described. One describes the preparation of the free base, and two of its trihydrochloride.1. A five stage synthesis (Scheme 1) which gave14C‐donetidine (8) labelled in the C2position of the pyrimidone ring starting from barium [14C]cyanamide. The overall radiochemical yield for the synthesis was 9% to give (8) at a specific activity of 57.8mCi/mmol.2. A three stage synthesis (Scheme 2) which gave14C2‐donetidine trihydrochloride (14) labelled in both methylenes of the aminoethylthio moiety starting from [1,2‐14C2]‐cysteamine hydrochloride. The overall radiochemical yield for the synthesis was 18% to give (14) at a specific activity of 15.4mCi/mmol.3. A five stage synthesis (Scheme 3) which gave3H‐donetidine trihydrochloride (22) labelled in the methylene of the furanlmethylthio moiety starting from sodium boro[3H]‐hydride. The overall radiochemical yield for the synthesis was 1% to give (22)