Control rats and rats with adjuvant-induced arthritis (AA) were given sulphadimidine (40 mg/kg body weight, i.v.). The absolute amount of total sulphadimidine (unchanged + acetylated) eliminated in urine was similar in both sexes of control rats, females, having, however, a higher proportion of acetylsulphadimidine (Ac-S) than males (on average 64% and 46%, respectively). Marked promotion of elimination of sulphadimidine to urine was found after adjuvant treatment in both sexes (by 52% in females and 64% in males). The main component of this excess was Ac-S, representing 99.6% of the increased amount in males, and 74.1% in females. Due to the developing hypoalbuminemia, blood protein-bound sulphadimidine decreased from the control value of 74% to 60% in AA animals. This change had no immediate relation, however, to the elevated production and elimination of Ac-S. It is suggested that as contrary to the impairment of microsomal drug metabolism, mycobacterial adjuvant does not impair, but even stimulates the extra-microsomal metabolism, at least as far as the acetylation is concerned.