Amikacin (AMK) was administered mainly to neonates by either intravenous drip infusion or intramuscular injection and its pharmacokinetic changes were investigated. The results obtained are summarized as follows. 1. Serum half-lives of AMK in neonates at ages 0 to 3 days were longer than those at ages 4 to 10 days. Serum half-lives were prolonged particularly in neonates at an age 0 day. Neonates at ages 11 to 15 days, also showed longer half-lives in comparison to infants. Similar peak serum levels were observed in all the neonates with ages 0-15 days. 2. Similar serum AMK levels were obtained in neonates through intravenous drip infusion and through intramuscular injection at a same dose level. 3. When the drug was administered to neonates at 3.0 to 6.0 mg/kg by intravenous drip infusion, peak serum levels did not reach 30 micrograms/ml which is considered to be the critical level for AMK to be toxic. 4. Urinary excretion rates in neonates 11 day or older were almost the same levels as in infants. 5. AMK, when administered through intravenous drip infusion, was observed to have a higher migration rate to saliva when compared with kanamycin administered through intramuscular injection. 6. Based on the results obtained from the present study, the following doses seem to be optimal for neonates, but further studies are required to be conclusive. For neonates: 2.0 to 5.0 mg/kg daily in 1 to 2 divided doses. (For those at ages of 0 to 3 days: 2.0 to 3.0 mg/kg) For infants: 3.0 to 8.0 mg/kg daily in 1 to 2 divided doses through intravenous drip infusion over a period of 30 minutes to 1 hour.