The effects of the macrolide antibiotics, erythromycin, clarithromycin and roxithromycin, on the pharmacokinetic profile of phenytoin were studied in rats. Animals were injected with phenytoin (100 mg/kg, i.p.) daily for 4 days and then they were given phenytoin (20 mg/kg, i.p.) alone or the same dose of phenytoin together with erythromycin (50 mg/kg, i.p.), clarithromycin (50 mg/kg, i.p.) or roxithromycin (50 mg/kg, i.p.). In another set of experiments, the same protocol was followed except that erythromycin (100 mg/kg), clarithromycin (100 mg/kg) and roxithromycin (100 mg/kg) were given by the oral route. The concentrations of phenytoin in plasma were determined using a high-performance liquid chromatographic method. The area under the curve the maximum plasma concentration and the elimination half-life (t1/2) of phenytoin were significantly (p < 0.05) increased by the macrolides. In addition, the macrolides significantly (p < 0.05) reduced the level of hepatic cytochrome P-450 in the rats. These results suggest that a potentially harmful drug-drug interaction may occur if phenytoin is administered concurrently with erythromycin, clarithromycin or roxithromycin.