The purpose of this study was to investigate the influence of chronic calcium treatment on the structure and function of thyroid C cells in ovariectomzed adult female rats. Eighteen 3-month-old, female Wistar rats were divided into three groups. The first group was used as the sham-operated control, and the other two were surgically ovariectomized (Ovx). One month after gonadectomy, one group of Ovx rats was injected with 28.55 mg Ca-glucoheptonate (Ca)/kg b.w., while the other two groups were chronically treated with vehicle alone (Ovx and sham control). Two months after surgery, the animals were killed. In the thyroid C cells, calcitonin (CT) was localized with the peroxidase-antiperoxidase method. Stereology was used to evaluate morphometric changes in the volume of C cells, their nuclei and relative volume density. The number of C cells per unit area was calculated. Serum CT content was determined by radioimmunoassay. After chronic Ca treatment C cells were numerous with darker cytoplasm than in C cells of sham-operated control animals, but more degranulated than the corresponding cells of Ovx rats. Their volume was significantly decreased by 14% (p < 0.05), while the number was increased by 47% (p < 0.05) in comparison with corresponding controls. Serum CT concentration was decreased by 27% (n.s.) in comparison to sham-operated control. Calcium treatment of Ovx rats led to a 32% increase of serum CT concentration in relation to untreated Ovx animals. These results suggest that chronic Ca treatment of Ovx female rats positively affected CT release from thyroid C cells, without any significant changes in morphometric parameters.