A "cocktail"of several probe drugs is often used to evaluate metabolic activity of multiple cytochrome P450 enzymes in one session. Some interactions among probe drugs can appear and may impact the rate of biotransformation of other ones. Our presented work was aimed on the influence of bupropion on cytochrome P450-mediated metabolism of tolbutamide. The biotransformation rates of tolbutamide administered either separately or in combined with bupropion were compared in this study. The results revealed that bupropion had no significant effect on tolbutamide hydroxylation. Thus, due to stability in cytochrome P450 enzyme metabolic activity in the case of combining of two model probe drugs the procedure can show to no extent differential results comparing to the single-marker use.