Immunoreactivity to endogenous calcitonin was studied in osteoblasts from calvaria of neonatal rats, by immunocytochemistry with the use of ultrathin sections obtained by cryoultramicrotomy. The influence of a silicon-treatment on the localization of endogenous calcitonin in osteoblasts of rats, was tested. Endogenous calcitonin-like immunoreactivity was observed in osteoblasts of calvaria of control and of silicon-treated rats, and showed a similar subcellular localization in both types of rats. Calcitonin was observed at the plasma membrane level, in the cytoplasmic matrix, and in the nucleus. The results provide immunocytological evidence for: 1) the internalization of calcitonin in osteoblasts of rat calvaria; 2) a similar localization of calcitonin in osteoblasts of control or silicon-treated rats.