The effect of supplementation with daily doses of vitamin D2 (1000 IU or 3000 IU, 25-75 micrograms, 63-189 nmol) has been studied in 39 premature neonates, initial gestational age 25-32 weeks. The initial mean plasma 25-hydroxyvitamin D was 25.8 nmol/l (10.3 ng/ml) but in 12 infants, most of whom were born in the winter months, the level was less than 15 nmol/l (6 ng/ml), and in seven babies plasma 1,25-dihydroxyvitamin D was less than 48 pmol/l (20 pg/ml). These findings suggest a considerable degree of maternal vitamin D-deficiency. Maximum attained concentrations of 25-hydroxyvitamin D on treatment were 77.3 nmol/l (30.9 ng/ml), high dose and 86.8 nmol/l (34.7 ng/ml), low dose, the mean rate of increase was greatest during the first two weeks (2.2 nmol/l/d; 0.88 ng/ml/d) and declined over the next 4 weeks. Mean maximum concentrations of 1,25-dihydroxyvitamin D2 were 283 pmol/l, (121 pg/ml), high dose and 309 pmol/l (129 pg/ml), low dose. Apart from a minor contribution to the initial plasma 1,25-dihydroxyvitamin D concentration, no effect of gestational age could be discerned in any of the measured variables. The endogenous pool of 1,25-dihydroxyvitamin D3 decayed with a T 1/2 of 22.5 d, indicating that vitamin D supplementation of these infants was necessary to avoid vitamin D-deficiency.