Proton pump inhibitors (PPIs) are the cornerstone in the treatment of gastresophageal reflux disease (GORD). PPIs are metabolized by the hepatic cytochrome P-450 enzymes (CYP-450). Rabeprazole is a PPI whose metabolism shows fewer interactions compared to other PPIs. In this study we evaluated the influence of rabeprazole administration on hepatic CYP-450 activity as measured by the (13)C-aminopyrine breath test ((13)C-ABT) in a group of patients with GORD. (13)C-ABT was performed on five GORD patients both before and after 1 week of rabeprazole administration (20 mg, b.i.d.). Pretreatment (13)C-ABT results were compared to posttreatment results. Pre- and posttreatment (13)C-ABT results for patients were compared to those obtained in five controls who did the test twice, with a 1-week interval in between. Before treatment, the (13)C-ABT results for the GORD patients did not significantly differ from those of healthy subjects. After treatment, we observed no significant modification of the (13)C-ABT in GORD patients compared to pretreatment values ((13)C-ABT %dose/hr, 10.56+/-1.31 versus 11.17+/-0.88; (13)C-ABT %cumulative dose, 8.08+/-1.11 versus 8.34+/-0.56). Posttreatment (13)C-ABT results were not significantly different from those obtained in controls at weekly repetition of the test. In patients with GORD, 1-week, full-dose rabeprazole does not display any significant interactions with CYP-450 activity.