Intravenously co-administered ketoprofen decreased the plasma concentration of sulpha-dimethoxine (SDM) after intravenous bolus administration to fast acetylator rabbits, and significantly increased the total body clearance (CLtot) and steady-state volume of distribution (Vdss) of SDM. On the other hand, ketoprofen had little effect on the plasma concentration of SDM in slow acetylator rabbits. When SDM was intravenously administered in combination with ketoprofen, an increase in the plasma concentration of N4-acetylsulphadimethoxine, a major metabolite of SDM that strongly displaces SDM from its binding sites, was observed in all rabbits, but the increase was much larger in fast acetylators. We conclude that the acetylation capacity for SDM is a factor determining the pharmacokinetic interaction between SDM and ketoprofen in rabbits.