Immunoreactivities to endogenous calcitonin, endogenous parathyroid hormone, endogenous estradiol and estradiol receptors were studied in osteoblasts from the calvaria of neonatal mice by immunocytochemistry with the use of ultrathin sections obtained by cryo-ultramicrotomy. Tissues were fixed in glutaraldehyde, postfixed in osmium tetroxide and frozen in liquid nitrogen. Estradiol and estradiol receptors could not be detected in osteoblasts, whereas calcitonin- and parathyroid hormone-like immunoreactivities were observed in this cell type. Calcitonin and parathyroid hormone had similar subcellular localizations: immunoreactivities were observed at the plasma-membrane level, in the cytoplasmic matrix, and in the nucleus. These results provide immunocytological evidence for: 1) the internalization of calcitonin and parathyroid hormone in osteoblasts; 2) a direct participation of calcitonin and parathyroid hormone in the regulation of osteoblasts; 3) the absence of estradiol receptors and estradiol in osteoblasts.