The effects of erythromycin on the pharmacokinetics and pharmacodynamics of glyburide were evaluated in 12 patients with non-insulin-dependent diabetes mellitus (fasting blood glucose levels 140-280 mg/dL), who received 4 days of treatment with erythromycin base (333 mg administered orally every 8 hr) and a control treatment in a randomized crossover design; 5 mg glyburide was administered on day 4 of each study period. Serum glyburide concentrations were determined by high-performance liquid chromatography. Peak serum glyburide concentrations were increased by 18%, and mean time to peak glyburide concentrations (Tmax) decreased from 4.9 to 3.0 hours during erythromycin treatment; only the difference in Tmax was statistically significant. No significant effects on glyburide clearance were observed. No significant differences in glucose clearance after carbohydrate loads were observed between erythromycin + glyburide and glyburide treatments. These data show that oral erythromycin base treatment does not affect glyburide metabolism but does affect the rate of glyburide absorption. This effect may be mediated by the stimulation of gastric motility by erythromycin. The clinical significance of the effects of erythromycin on glyburide kinetics appears to be minimal, based on the determinations of serum glucose concentrations.