The induction of liver drug metabolism was investigated in five patients before and after the administration of 800 mg sulfinpyrazone daily for 4 weeks, by using antipyrine plasma-pharmacokinetics and by determining urinary excretion of 6-beta-OH-cortisol and serum gamma-glutamyl-transpeptidase (GGT) activity. Antipyrine half-life was shortened in all patients from a mean value of 12.3 +/- 3.9 h to 7.8 +/- 2.0 h and antipyrine clearance was increased from 39.0 +/- 16.0 ml/min to 57.6 +/- 13.7 ml/min. In contrast the volume of distribution of antipyrine was unaffected; the values being 38.0 +/- 8.6 liters and 37.4 +/- 5.7 liters, respectively. In all patients the excretion of 6-beta-OH-cortisol in the urine went up from 65.0 +/- 25.7 micrograms/24 h to 346.8 +/- 193.4 micrograms/24 h. The ratio 6-beta-OH-cortisol/free cortisol changed from 4.1 to 15.8. After 21 days of treatment the GGT increased from 17.4 +/- 4.9 units/liter to 32.6 +/- 12.5 units/liter The data presented confirm that sulfinpyrazone induces drug metabolism in patients of the older age group. Interactions between sulfinpyrazone and other drugs given simultaneously must be borne in mind.