To assess the molybdenum supply and requirements of preterm infants, Mo concentration was determined in milk from mothers of 6 term and 11 preterm newborns; no difference was found between fore- and hindmilk and no diurnal variations were found during 24-h collections. Respective values (means +/- SD) of term and preterm milks were 10.2 +/- 3.7 and 4.0 +/- 3.7 micrograms/L (106.2 +/- 38.5 and 41.7 +/- 38.5 nmol/L) at 3-5 d of lactation, 4.8 +/- 3.9 and 3.7 +/- 3.8 micrograms/L (50.0 +/- 4.6 and 38.5 +/- 39.6 nmol/L) at 7-10 d, 1.5 +/- 1.4 and 1.4 +/- 0.9 micrograms/L (15.6 +/- 14.6 and 14.6 +/- 9.6 nmol/L) at 14 d, 2.6 +/- 2.2 and 1.9 +/- 1.4 micrograms/L (27.1 +/- 22.9 and 19.8 +/- 14.6 nmol/L) at 1 mo, and 0.2 and 1.2 +/- 0.5 micrograms/L (2.1 and 12.5 +/- 5.2 nmol/L) at 2 mo. A statistical difference was found between term and preterm milk at 3-5 d of lactation. During lactation significant changes were found between the periods 3-5 d and 7-10 d, 14 d, 1 mo (p less than 0.01) and 2 mo (p less than 0.05) of lactation and between 7-10 d and 14 d (p less than 0.05). According to the requirements of the preterm infant, a supplementation of 2-3 micrograms.kg-1.d-1 by enteral route is suggested.