Serine occupies a central position in folate-dependent, one-carbon metabolism through 5,10-methylenetetrahydrofolate (MTHF) and 5-formyltetrahydrofolate (FTHF). We characterized the ontogeny of the specific activity of key enzymes involved in serine, 5,10-MTHF, and 5-FTHF metabolism: methenyltetrahydrofolate synthetase (MTHFS), MTHF reductase (MTHFR), the glycine cleavage system (GCS), methionine synthase (MS), and serine hydroxymethyltransferase (SHMT) in rabbit liver, placenta, brain, and kidney. In liver, MTHFS activity is low in the fetus (0.36 +/- 0.07 nmol. min(-1). mg protein(-1)), peaks at 3 wk (1.48 +/- 0.50 nmol. min(-1). mg protein(-1)), and then decreases to adult levels (1.13 +/- 0.32 nmol. min(-1). mg protein(-1)). MTHFR activity is highest early in gestation (24.9 +/- 2.4 nmol. h(-1). mg protein(-1)) and declines rapidly by birth (4.7 +/- 1.3 nmol. h(-1). mg protein(-1)). MS is highest during fetal life and declines after birth. Cytosolic SHMT activity does not vary during development, but mitochondrial SHMT peaks at 23 days. GCS activity is high in the fetus and the neonate, declining after weaning. In placenta and brain, all activities are low throughout gestation. Cytosolic and mitochondrial SHMT activities are low in kidney and rise after weaning, whereas MTHFS is low throughout development. These data suggest that the liver is the primary site of activity for these enzymes. Throughout development, there are multiple potential sources for production of 5,10-MTHF, but early in gestation high MTHFR activity and low MTHFS activity could reduce 5,10-MTHF availability.