The deuteriation and tritiation of a number of drugs containing carboxyl, amide, aralkylamine, and anilide functional groups have been investigated using homogeneous rhodium trichloride as a catalyst. Good incorporation of deuterium was observed and the regiospecificity for ortho exchange was very high for most of the drugs studied. Similarly, with tritium, good incorporation (specific activities) and regiospecificities were achieved in many cases. Satisfactory results were also obtained from the small number of heterocyclic-containing drugs that were included in the present study.