Blood glucose, plasma insulin and plasma glucagon were determined during an arginine test at 0, 30 and 60 min in 1 to 7 and 30 day infants. The results were compared with those obtained in the 2-10-year-old children. Basal levels of blood glucose increased from the 1st to the 30th day; by the 30th day they were comparable with those observed in the 2-10-year-old children. Basal levels of plasma insulin were higher in 1 day infants, lower in 7 day infants and similar to those of the 2-10-year-old children in the 30 day infants. Basal values of plasma glucagon were higher in 1 day infants than in the 2-10-year-old children; the lowest levels were found in 7 day infants. After arginine, blood glucose increased in the 2-10-year-old children, the variations were negligible in the 1 to 7 and 30 day infants. Plasma insulin increased in 2-10-year-old children, there was a slight increase in 1 and 30 day infants and a negligible one in the 7 day infants. Plasma glucagon increased in the 2-10-year old children, and was similar in 1 day infants; the increase was reduced in 30 day infants and virtually absent in 7 day infants. These results suggest that there is a transitory alpha cell insensitivity to arginine during the first week of life.