Two consecutive studies of amikacin plasma levels were performed in 63 and 64 neonates whose postconceptional age (PCA) ranged from 26 to 45 weeks. The first study, using a dose of 7.5 mg/kg every 12 hours, permitted to establish a dosage related to PCA. Half-life elimination of amikacin was strongly correlated with PCA (r = -0.42, p = 0.0009). The apparent volume of distribution was correlated with none of the variables studied. The mean volume of distribution was 640 +/- 190 ml/kg. From these data, the inferred dosage was 10 mg/kg, given at intervals of time predetermined as a function of PCA. A second prospective study proved the value of this PCA adapted dosage. The maximal concentrations obtained were higher (21.6 +/- 5.9 vs 18.5 +/- 4.6 micrograms/ml, p less than 0.001), the nadir concentrations were not significantly increased (5.7 +/- 3.1 vs 5.2 +/- 3.7 micrograms/ml, NS) and the number of nadir concentrations outside the desired interval of 2-8 micrograms/ml was smaller (p less than 0.01).