The plasma concentrations of calcium, inorganic phosphorus, and magnesium were studied during the early postnatal period in a rodent (rats), in ruminants (lambs and calves), and in foals. Decreases in plasma calcium after birth were observed only in newborn rats and foals. In rats the postnatal fall in plasma calcium level was already evident 1 hr after cesarean section (9.20 mg/100 ml) as opposed to 11.90 mg/100 ml in utero) and reached a nadir within 6 hr (-4.05 mg/100 ml). Newborn foals showed a small decrease in plasma calcium (-0.73/100 ml) 48 hr after birth. In the four species plasma inorganic phosphorus increased in the days after birth; plasma magnesium levels were only slightly affected over the first week of life. In ruminants and in foals, the high plasma calcitonin levels were not always related to plasma calcium, nor with plasma inorganic phosphorus over the first week of life. Newborn calves showed at birth detectable levels of plasma parathyroid hormone, similar to those found in their mothers, with individual values ranging from 0.80-3.50 ng/ml. These results are discussed in light of the well known early neonatal nypocalcemia occurring in normal human newborns.