Acute maternal alterations in plasma calcium were used as an indirect mean to study the placental transfer of calcium in thyroparathyroidectomized (TPTX) pregnant rats. Calcium infusions to normal or TPTX females showed in both cases a saturation mechanism reflected on fetal plasma calcium. With TPTX mothers saturation curves of the fetal plasma calcium were similar whether fetuses were decapitated (deprived of brain, hypophysis, and thyroparathyroid complex) or intact. Increments in fetal plasma calcium were 4-fold higher in TPTX mothers (approximately 2.5 mg/dl), and 2-fold higher in thyroidectomized (TX) mothers (approximately 0.5 mg/dl). From the whole results, it appeared that increments in fetal plasma calcium secondary to maternal calcium infusion were probably related to the basal plasma calcium in the fetus, and not to the hormonal deficiency of the mother.