The effects of acute administration of torasemide (1-isopropyl-3-([4-(3-methyl-phenylamino)pyridine]-3-sulfonyl)urea), a new potent loop diuretic, were compared with those of furosemide in 8 healthy male volunteers (aged 22-35 years). The subjects were studied on two separate occasions, at 1-week intervals. After two baseline urine collections of 30 min, either torasemide (20 mg i.v.) or furosemide (40 mg i.v.) was given, followed by further timed urine collections after 15, 40 and 60 min. There were significant increases in urine flow, and in sodium, potassium, chloride and calcium excretion, which were maximal within 15 min after administration of both drugs. These effects were of similar magnitude except for the natriuretic and chloruretic actions which were more pronounced after furosemide than after torasemide (p less than 0.02). The drugs induced no significant changes in effective renal plasma flow or glomerular filtration rate, as measured from the clearances of constantly i.v. infused 123I-orthoiodohippuran and 51Cr-EDTA, respectively. Urinary dopamine, urinary prostaglandin E excretion and plasma renin activity were increased by both drugs. It is concluded that the acute effects of torasemide are comparable to those of furosemide.