Antipyrine plasma clearance and rates of metabolite formation were measured on four occasions in eight patients with disseminated nonseminomatous testicular cancer. Antipyrine tests were performed before, during (2X), and after treatment with a combination of cisplatin (P), vinblastin (V), and bleomycin (B). Pretreatment values were compared with a male control group (n = 14) matched for age and body weight. Antipyrine plasma clearance was 20% higher in patients with testicular cancer (first experiment) than in the control group. This difference was mainly due to a 35% higher clearance for production of 3-hydroxymethylantipyrine (HMA), while clearance for production of norantipyrine (NORA) and 4-hydroxyantipyrine (OHA) was not significantly different from the control group. A reduction in CLHMA was observed after complete remission (fourth experiment), indicating that the presence of the tumor may be related to a selective increase of HMA formation. Treatment with the PVB combination resulted in a 30% increase in antipyrine plasma clearance (second and third experiments), whereas the rates of formation of the main metabolites of antipyrine were all increased to the same extent. These accelerating effects of PVB treatment persisted for at least 6 weeks after the start of the last treatment cycle. The data presented in this paper demonstrate that the presence of a testicular tumor and the use of cytostatics can have an accelerating and partially selective effect on oxidative drug-metabolizing enzyme activity in man.