Absorption of phenobarbital after the intramuscular administration of single doses in infants.

Blood concentrations of phenobarbital, following an intramuscular administration of a single large dose of approximately 10 mg/kg, were studied in 39 infants. A rapid rise was obtained with a mean concentration of 9.30 mug/ml at 30 minutes, 12.76 mug/ml at 90 minutes, and a mean peak concentration of 13.28 mug/ml, which was reached in most cases within 2 hours of injection or less. Cerebrospinal fluid values in 13 patients averaged half the blood phenobarbital concentrations.