We studied the rate of sulfamethazine acetylation in athletes and untrained controls aging 18-22 years. The rate of sulfamethazine acetylation in controls was characterized by a bimodal distribution: rapid and slow acetylators constituted 42 and 58%, respectively. The rate of sulfamethazine biotransformation in athletes was characterized by a trimodal distribution: ultrarapid, rapid, and slow xenobiotic acetylators constituted 48.4, 22.6, and 29%, respectively. The ultrarapid acetylation phenotype was probably associated with N-acetyltransferase induction and reflected adaptation to physical exercises