In 89 healthy full-term newborn infants, we studied the different contribution of gastrin 17 (G17) and gastrin 34 (G34) to neonatal hypergastrinemia and the G17 and G34 response to a meal in the first days of life. Serum concentrations of G17 and G34 were measured by radioimmunoassay specific for the NH2-terminus of G17 and G34 in 23 newborn infants in the cord blood and in 66 newborn infants before or 20 min after bottle-feeding. Basal serum G17 and G34 values were also obtained in 38 healthy fasting adults. Mean (+/- SEM) G17 levels in the cord blood were not different from those of the adult controls (29.28 +/- 4.16 versus 31.00 +/- 2.62 pg/ml) and increased significantly either at 12 h (48.06 +/- 7.32 pg/ml, p less than 0.025) or on the 4th day of life (80.56 +/- 9.99 pg/ml, p less than 0.01). Serum G34 levels in the cord blood were significantly higher than in adult controls (163.22 +/- 11.19 versus 126.68 +/- 5.57 pg/ml, p less than 0.005) and increased at 12 h of life (225.22 +/- 25.95 pg/ml, p less than 0.02), but no increase was found on the 4th day of life (204.87 +/- 18.08 pg/ml). Neither postprandial G17 nor G34 increases were found on the 1st or on the 4th day of postnatal life. The study supports the following conclusions: (a) neonatal hypergastrinemia is mainly due to G34 fraction; (b) the increased levels of gastrin on the 4th day of life are due to G17 fraction; (c) bottle-feeding does not stimulate either G17 or G34 release in the first 4 days of life.