1. Leucocyte ascorbic acid concentrations have been measured in 1147 females during early pregnancy and in smaller numbers of women before conception, throughout pregnancy and at 6 months post partum.
2. The leucocyte concentration in the 1st trimester was found to be affected by season, social class and smoking. Selecting individuals by extremes of social class, season and smoking produced two small populations with almost separate ascorbic acid distributions and mean concentrations of 21.7 and 45.1 μg/108 leucocytes.
3. Early pregnancy had little effect on leucocyte ascorbic acid concentrations but values decreased in the second trimester. However, this was associated with a leucocytosis so that the total leucocyte ascorbic acid content of blood was unchanged.
4. Low ascorbic acid concentrations during the 1st trimester were not associated with subsequent spontaneous abortions, still-births or neonatal deaths, but there was an increased frequency of low values in women who gave birth to infants smaller than 3250 g.
5. The adequacy of ascorbic acid reserves in early pregnancy is discussed.