The role of vitamin D (D) in fetal mineral metabolism remains to be established. We have studied transplacental transfer, fetal distribution and metabolism of 3H-1,25(OH)2D (3H-1,25) in 5 Rhesus monkeys during the latter third of pregnancy. Indwelling catheters were placed in maternal (mat.) aorta, uterine vein, and fetal umbilical artery & vein. Serial blood was obtained from these sites over 3 hrs. Amniotic fluid and mat. urine were collected every 60 min. Serum, urine, amniotic fluid, and various tissue samples were extracted and analyzed by high performance liquid chromatography to determine total radioactivity (RA) and specific D metabolites. Peak RA levels in the fetus were reached within 20 min post injection. RA concentrations in fetal serum were 10-15% of mat. serum levels. RA concentrations in amniotic fluid and fetal urine were 10% of fetal serum concentrations. 75-85% of mat. & fetal serum RA was identified as intact 3H-1,25. Up to 50% of tissue RA consisted of 3H-1,25 in a fetus delivered 3 hrs after dose administration. However, in fetuses aborted 24-48 hrs post injection, no 3H-1,25 but highly polar compounds were found. These data suggest that 1,25(OH)2D crosses the placenta in subhuman primates as has been shown for other sterols. Furthermore, the fetus appears to be capable of converting 1,25(OH)2D to more polar steroid derivatives.