A study of the effects of intravenous administration of either 150 mg or 250 mg of furosemide to standardbred mares pre-treated with other drugs was undertaken to determine whether a unique pattern of drug elimination into urine and from plasma for each compound occurred. Furosemide significantly reduced the plasma concentrations of codeine compared to control 2-6 h after furosemide administration. In contrast, the plasma concentrations of theophylline, phenylbutazone, pentazocine, guaifenesin and flunixin were not markedly altered by furosemide. In the case of acepromazine, clenbuterol and fentanyl, the data generated were insufficient to state with certainty whether or not furosemide affected the plasma concentrations of these three drugs. A significant reduction was noted in the urinary concentrations of guaifenesin, acepromazine, clenbuterol, phenylbutazone, flunixin, fentanyl and pentazocine within 1-4 h of furosemide administration. The urinary concentrations of theophylline remained reduced as long as 8 h after furosemide injection. Furosemide administration to horses pre-treated with codeine resulted in depression of urinary morphine concentrations 2-4 h and 9-12 h after furosemide injection. A lower furosemide dose (150 mg) produced changes in drug urinary excretion and plasma elimination equivalent to the higher dose (250 mg). It is evident that furosemide affects the urinary and plasma concentrations of other co-administered drugs but not in a predictable fashion, which limits the extrapolation of these results to as yet untested drugs.