A pharmacokinetic study was conducted in neonates (mature, premature) to which amikacin (AMK) was administered through intravenous drip infusion. The results of the study are summarized below. 1. Changes in blood concentrations of AMK obtained after intravenous drip infusion over a period of 30 or 60 minutes were comparable to those after intramuscular injection. 2. When AMK was administered to neonates (mature, premature) at a single intravenous (30 or 60 minutes) dose of 6 mg/kg, peak levels of 15.5 to 26.3 micrograms/ml were attained. These values were within the range of 15 to 30 micrograms/ml which are considered to be safe and effective peak levels. 3. In 0 day-neonates, half-lives of blood AMK levels rather long and widely varied (3 to 8 hours) but, in about 7 day-neonates, half-lives were 3 to 4 hours. 4. It is considered from the above results that the safe and effective blood concentrations of AMK in 0 to 7 day-old neonates can be obtained from intravenous administrations at each dose of 6 mg/kg repeated with intervals of 12 or 24 hours and, in 8 days or older neonates, from intravenous drip infusions over 30 or 60 minutes at each dose of 6 mg/kg repeated with intervals of 12 hours. 5. For neonates with very low birth weights, individual doses and intervals should be separately investigated.