The effects of sulphinpyrazone administration on the metabolic and renal clearances of theophylline in man were investigated. Sulphinpyrazone increased total plasma theophylline clearance by 22%. This was the sum of increases in metabolic clearances by 3-demethylation (32%), 1-demethylation (30%) and 8-oxidation (22%) and of a decrease in renal clearance (27%). It is proposed that sulphinpyrazone has differential effects on at least two forms of cytochrome P-450 inhibiting one enzyme or group of enzymes which metabolises tolbutamide, phenytoin and S(-)warfarin and inducing a form (or forms) which metabolises theophylline, antipyrine and R(+)warfarin.