In 14 neonates 1-4 weeks old, 30 babies aged 1-12 months, and 7 infants of 1-5 years of age, the serum levels of phenobarbital were determined by a gas chromatographic micro-method after intravenous injection of phenobarbital 5-10 mg per kg body weight. It was possible to calculate the pharmacokinetic parameters using a two compartment open model. The distribution volumes within the individual age groups and the rate constants k12 and k21 showed no significant differences, but the elimination half-life was significantly longer in neonates (118.6+/-16.1h) than in babies (62.9+/-5.2h) or infants (68.5+/-3.2h).