The influence of 1,25-dihydroxycholecalciferol (0.05 microgram/kg bw/day, given I.M. for 10 days) on calcium transfer from dam to foetus and on foetal calcium and phosphate content was observed in 3 groups of 10 guinea pigs, one group on each of days 40, 50 and 66 of gestation. At 40 and 50 days of gestation, 1,25-dihydroxycholecalciferol increased the calcium and phosphate content of the foetus. 45Ca transfer from the dam to placenta (measured during a 30-90 min period) was higher in treated than in control animals.