The influence of sulfamethizole on the metabolism of diphenylhydatoin (DPH) tolbutamide, and warefarian is examined. In 8 patients DPH means half-life (T/2) increased from 11.8 plus or minus 3.6 hr to 19.6 plus or minus 5.2 hr and mean metabolic clearance rate (MCR) decreased from 43.7 plus or minus 16,8 to 28.1 plus or minus 9.1 ml/min durus or minus 1.2 to 9.2 plus or minus 1.2 hr MCR decrease from 17.0 plus or minus 5.4 to 10.5 plus or minus 1.2 ml/min. In 2 patients warfarin T/2 increased fron an average of 64.7 to 92.7 hr and MCR decreased from 1.65 ml/min to 1.05 ml/min. In 4 patients on long-term DPH treatment after 1 wk on sulfamethizole inhibits hepatic metabolism of DPH, tolbutamide, and warfarin.