[en] Mapindolol, a potent β-adrenoceptor blocking agent, was labelled with 14C by two different routes yielding the 2-[14C]methyl and [2-14C]products. While the first alternative was abandoned because of low overall yield, the latter route provided mepindolol with a specific activity of 2.06 GBq/mmole (55.8 mCi/mmole) and in a radiochemical yield of 40% starting from Ba14CO3. (author)