The vitamin D-binding protein (DBP) was measured in rat serum using a single radial immunodiffusion technique. Normal serum levels at birth (74 +/- 11 mg/liter, mean +/- SD) were lower than during the last day of fetal life (130 +/- 14 mg/liter) and much lower than in adult rats. A marked sex difference in DBP occurred after puberty: male values (656 +/- 52 mg/liter) were significantly higher than female values (472 +/- 46 mg/liter). The sex difference could be abolished by either adult gonadectomy or transpharyngeal hypophysectomy. Implantation of a pituitary gland under the renal capsule in hypophysectomized male rats further decreased the DBP concentration, suggesting that PRL suppresses the DBP level. A similar decrease was also observed at the end of pregnancy and during lactation. Administration of androgens to either normal female or gonadectomized male rats increased their DBP concentration to the normal adult male level. The serum levels of total 25-hydroxyvitamin D did not fluctuate according to the concentration of DBP, indicating that the concentration of "free 25-hydroxyvitamin D" is not regulated at a constant level.