Fasting serum gastrin concentrations were determined by radioimmunoassay for 124 infants and children free of gastrointestinal disease. Values for the entire group ranged from 1.1 to 167.9 pg/ml (mean, 32.3 pg/ml). Mean serum gastrin levels for children who fasted eight or more hours were 22.5+/-23.9 pg/ml while the mean serum gastrin levels for children who fasted four to eight hours ranged from 42.4+/-33.9 pg/ml to 59.2+/-49.6 pg/ml. The mean serum gastrin levels varied inversely with body surface area among children who fasted the same length of time.