Fine structural alterations of thyroid C cells and parathyroid chief cells were evaluated after feeding dried leaves of the calcinogenic plant, Solanum malacoxylon, to cattle for 1, 6 and 32 days. Thyroid C cells initially were degranulated in response to the hypercalcemia, and parathyroid chief cells accumulated secretory granules. There was hypertrophy of thyroid C cells with well-developed secretory organelles but few secretory granules in the cytoplasm after 6 days of feeding S. malacoxylon. Inactive chief cells with dispersed profiles of endoplasmic reticulum and increased lysosomal bodies predominated in the parathyroid glands. Multiple foci of soft tissue mineralization were present in the heart, lung, and kidney. Thyroid C cells underwent hypertrophy and hyperplasia after 32 days of S. malacoxylon, and parathyroid chief cells were inactive or atrophic in response to the long-term hypercalcemia. Severe soft tissue mineralization was present throughout the cardiovascular system, lung, kidney, and spleen. These ultrastructural changes in thyroid C cells and parathyroid chief cells plus the widespread soft tissue mineralization observed after feeding cattle small amounts of S. malacoxylon are consistent with the recent evidence that leaves of this plant are a potent source of the active metabolite, 1,25-dihydroxycholecalciferol, of vitamin D.