Pharmacokinetics of netilmicin (NTL), a new aminoglycoside antibiotic, injected intramuscularly to Beagle dogs were compared with those of gentamicin (GM), and the relationship between NTL dosage administered and plasma level and urinary excretion was examined. 1. When administered to 6 male Beagle dogs at a dose of 10 mg/kg, the plasma level showed a broad peak (ca. 23 mcg/ml) at 10 to 45 minutes after administration and declined thereafter with half-life of 65 minutes. On the other hand, the plasma level of GM administered at a dose of 10 mg/kg showed a peak (ca. 24mcg/ml) at 15 to 45 minutes after administration and the elimination half-life was 75 minutes. Both of the urinary concentrations of NTL and GM administered at a dose of 10 mg/kg were highest in the 4- to 6-hour urine, whereas NTL and GM recovered in the 24-hour urine were 51.7% and 57.7% of the dose administered, respectively. The pharmacokinetic profile of NTL administered intramuscularly to Beagle dogs appeared to be almost identical to that of GM. 2. The peak plasma level of ca. 50 mcg/ml was obtained at 10 to 60 minutes after administration of NTL at 20 mg/kg, and the half-life was 74 minutes. NTL recovered in the 24-hour urine was 69.7% of the dose administered. 3. TLC-bioautograms showed no biologically active metabolites of NTL in the urine collected from Beagle dogs given the antibiotic intramuscularly.