Plasma 25-hydroxycholecalciferol (25OHD3) has been measured by means of gas-liquid chromatography (HPLC) in 23 healthy mothers and their normal full term babies. It was measured as well in twelve infants, nine children of 1.5-5 years and in 57 schoolboys of 5-12 years. Infant population showed a wide variation of plasma 25OHD3 (m +/- SEM) being 16.4 +/- 1.45 ng./ml. Children 1.5-5 years old showed 12.7 +/- 2.3 ng./ml. which is lower than expected. Schoolboys showed a normal values of 22.9 +/- 1.3 ng./ml. These differences are believed to be due to a great sunshine exposition. In perinatal group, births taking place in winter showed a very low level of 25OHD3 (m +/- SEM) in mothers (6.9 +/- 1.8 ng./ml.) and consequently in their newborns (5.3 +/- 1.6 ng./ml.). Those taking place in summer showed significantly higher figures for both mothers (18.9 +/- 2.7 ng./ml.) and newborns (12.9 +/- 2.4 ng./ml.). A good correlation (r = 0.83) between plasma levels of mothers and newborns is found.