The induction dose of thiopental and propofol has been shown previously to vary during childhood. The methohexital dose needed for satisfactory induction of anesthesia in 50% of patients (ED50) was determined in 75 infants and children, 1 month to 16 yr of age. An intravenous bolus of methohexital, dissolved in a lipid emulsion to decrease pain on injection, was given over 10 s. After 30 s the anesthesia mask was applied. The patient was considered to be asleep if there were no gross movements when the head was placed in the sniffing position and the anesthesia mask applied, and no response to verbal command (tested in children more than 4 yr of age) during the next 30 s while the patient breathed O2. ED50 (+/- SE) was 2.6 +/- 0.2 mg/kg in infants 1-6 months of age, 1.9 +/- 0.1 mg/kg in infants 7-11 months of age, 1.4 +/- 0.1 mg/kg in children 1-3 yr of age, 1.1 +/- 0.1 mg/kg in children 4-7 yr of age, and 1.3 +/- 0.1 mg/kg in children 8-16 yr of age. ED50 in each of the two groups of infants was significantly greater than ED50 in each of the three other groups (P less than 0.05). Pain or discomfort on injection was observed in 1 infant and 3 children (5%). Eight patients (11%) had apnea longer than 15 s, and excitatory phenomena occurred in 9 (12%). It is concluded that the dose of methohexital needed for induction of anesthesia varies with age.(ABSTRACT TRUNCATED AT 250 WORDS)