The effect of ibuprofen treatment (1600 mg day-1 per os for 7 days) on single dose (300 mg per os) dispositional parameters of phenytoin (DPH) was evaluated in 10 healthy males. Ibuprofen slightly increased the free fraction of DPH from mean (+/- s.d.) value of 6.9 (+/- 0.7)% to 7.8 (+/- 0.3)% (P less than 0.01). DPH total clearance (CL/F) was not significantly changed going from a value of 0.027 (+/- 0.009) 1 h-1 kg-1 to 0.032 (+/- 0.014) 1 h-1 kg-1 (0.05 less than P less than 0.1). Intrinsic unbound DPH clearance (CLuint/F) was unaffected by ibuprofen treatment. Neither DPH's apparent volume of distribution (V/F) nor the apparent volume of distribution of unbound DPH (Vunb/F) were significantly altered by ibuprofen treatment. The slight changes in DPH plasma protein binding that are due to concurrent ibuprofen treatment in doses of 1600 mg day-1 are not likely to produce important changes in DPH dispositional parameters.