Chloramphenicol, tetracycline, and sulfisoxazole were administered (orally) in separate trials to clinically healthy adult dogs of both sexes at 8-hour intervals for five consecutive 8-hour test periods. All urine was collected from each dog during each test period and an aliquot from each period was assayed for antimicrobial activity. Daily doses of the antimicrobics were as follows: chloramphenicol 99 mg/kg of body weight, tetracycline 55 mg/kg, and sulfisoxazole 66 mg/kg. Mean 8-hour urine concentrations (+/- 1SD) for chloramphenicol were 124 +/- 40 micrograms/ml; for tetracycline, 138 +/- 65 micrograms/ml; and for sulfisoxazole, 1,466 +/- 832 micrograms/ml. The mean 8-hour percentages of the doses of drug eliminated in active form in the urine were 6.3 +/- 2.6% for chloramphenicol, 11.2 +/- 2.0% for tetracycline, and 68.5 +/- 2.1% for sulfisoxazole.