Determinations of serum calcium (Ca), phosphorus (P), magnesium (Mg), calcitonin (CT) and parathyroid hormone (PTH) were carried out in full term newborn infants during the first 168 h of life. The infants were randomly assigned to two different diets: glucose and breast feeding (BF) only or early formula feeding (F). The Ca serum levels decreased from birth to the 24th h; after that time they increased until the 72nd h. The values of serum Ca at the 48th and 72nd h were significantly lower in the F than in the BF group. In contrast, the P serum levels increased from birth to the 48th h and then they decreased; the values of serum P at the 48th h were significantly higher in F than in the BF group. The CT serum levels increased from birth to the 24th h and then they decreased. No difference between the F and BF groups was found during the first 48 h, while at the 72nd h the F group demonstrated significantly higher values of serum CT. PTH serum levels also increased from birth to the 24th h with significantly higher values at the 72nd h in the F group. The results of this investigation demonstrate that the difference in feeding in the first hours of life affects the calcium homeostasis and the secretion of hormones involved in the regulation of serum Ca levels.