The influence of grapefruit juice (GFJ) on caffeine's metabolism and the hemodynamic effects of this potential food interaction were studied in 10 normotensive volunteers. In this crossover study, caffeine (3.3 mg/kg) and water or caffeine and GFJ were given to participants. Nine serum caffeine concentrations were determined within 24 hours of each phase. In another phase of this study, caffeine was given with multiple GFJ doses to 6 of the 10 participants. Ambulatory blood pressure (BP) monitors were used for 12 hours to assess treatment hemodynamic effects. The mean area under the serum caffeine concentration-time curve (AUC0-infinity) values +/- SD for the caffeine with water group, caffeine with GFJ group, and caffeine with multiple GFJ group were 47.0 +/- 10.8, 48.7 +/- 15.2, and 49.6 +/- 7.0 micrograms/ml.hr, respectively (NS). There was no significant difference on the ambulatory systolic BP, diastolic BP, percentage of the time with a diastolic BP greater than 90 mm Hg, or heart rate area under the effect curves. We conclude that grapefruit juice had no effect on caffeine pharmacokinetics or hemodynamic effects.