The effect of repeat oral doses of ritonavir, at high (600 mg twice daily) and low (100 mg twice daily) doses, on the pharmacokinetics of a single dose of bupropion was evaluated in healthy volunteers. Subjects received a single dose of 150 mg of bupropion on day 1 and twice-daily ritonavir from day 8 through day 30. Ritonavir was up-titrated from 300 mg twice daily to 600 mg twice daily in the high-dose ritonavir study, whereas subjects remained on 100 mg twice-daily ritonavir in low-dose ritonavir study. Subjects received a second single dose of bupropion on day 24. Serial blood samples were obtained to evaluate the pharmacokinetics of bupropion and its metabolites on days 1 and 24. Steady-state ritonavir led to a decrease of area under the curve and maximum plasma concentration of bupropion by 62% to 67% in the high-dose study and by 21% to 22% in the low-dose study, indicating a drug interaction of statistical and clinical significance, particularly at high doses of ritonavir. These studies demonstrate that the reduction of bupropion exposure by ritonavir is dose-related. Dosage adjustment of bupropion may be needed when administered with ritonavir. However, the maximum recommended daily dose of bupropion should not be exceeded.