Absorption, distribution, metabolism and excretion of isepamicin sulfate (HAPA-B) were studied following intramuscular, intravenous and drip intravenous administration at doses of 6.25, 25 and 100 mg/kg to rabbits. Plasma concentrations of HAPA-B following intramuscular, intravenous and drip intravenous administration depended on dose levels. Biological half-lives (T1/2), body clearance (Clt) and areas under plasma concentration-time curves (AUC) for different routes of administration were similar in all 3 routes. A theoretical curve for drug concentrations vs. time was obtained using pharmacokinetic parameters calculated from drug concentrations in plasma following a 45-minute drip intravenous administration. From the curve, it was estimated that 60 to 90 minutes would be required to achieve a similar maximum drug concentration in plasma by drip intravenous administration to that obtained by intramuscular administration. Thus, drug concentration patterns obtained following intramuscular administration could be duplicated in drip intravenous administration by regulating the length of time for infusion. The concentration of HAPA-B in tissues obtained following a 15-minute drip intravenous administration reached maximum after 15 minutes at a level higher than that achieved by intramuscular administration, but an hour later, concentrations in tissues including the kidney decreased to similar levels obtained following intramuscular administration and patterns of concentration decrease for drip intravenous administration and intramuscular administration were quite similar to each other thereafter. The drug was rapidly excreted into the urine following any of the 3 routes, and urinary recoveries in 24 hours were 75 approximately 92% of dose amounts for all dose levels tested. Bioautograms on thin-layer chromatographs of 0 approximately 6 hours urine samples obtained following an intramuscular administration of the drug showed a single biologically active bands with similar Rf values to HAPA-B itself. No active metabolite of the drug was detected in the urine.