The compatibility of furosemide with i.v. admixtures containing each of five different aminoglycosides was studied. Admixtures of amikacin 2 mg/ml, gentamicin 1.6 mg/ml, kanamycin 2 mg/ml, netilmicin 1.5 mg/ml, and tobramycin 1.6 mg/ml (as the sulfate salts) were prepared in both 5% dextrose injection and 0.9% sodium chloride injection in minibags. Furosemide injection 4 ml (40 mg) was then added to each admixture, and the admixtures were examined visually and microscopically for precipitate. The macroscopic and microscopic evaluations were repeated 15 minutes and 24 hours after mixing. To simulate Y-site injection of furosemide, furosemide injection 1 ml (10 mg) was added to 1 ml of each aminoglycoside admixture in a syringe. For admixtures in which precipitates formed, the pH was recorded before and after adding furosemide to subsequent admixtures and also after dropwise addition of 1N sodium hydroxide until the precipitate dissolved. Precipitates were identified using spectrophotometric analysis and melting point determinations. Addition of furosemide resulted in a precipitate only in admixtures containing gentamicin sulfate or netilmicin sulfate; the results for the simulated Y-site injection study were the same. Spectrophotometric analysis and melting point determinations revealed that the precipitate was furosemide. Because furosemide precipitates when added to admixtures containing either gentamicin sulfate or netilmicin sulfate in 5% dextrose injection or 0.9% sodium chloride injection, furosemide should be administered separately or the i.v. tubing should be flushed thoroughly before and after administering this drug via a Y-injection site.