Stereological methods were used on the light- and electronmicroscopic levels to estimate total volumes and surfaces of secretory cells and organelles in normal and hypocalcemia-stimulated rat parathyroid glands. Weanling rats were given a diet containing 0.02% Ca. After 28 days they had severe hypocalcemia and about a two-fold increase of serum levels of immunoreactive parathyroid hormone. The parathyroids were enlarged, mainly because of an increase of cytoplasmic volume of secretory cells. Volume and surface densities (fractions) of cells and organelles were, however, only moderately increased. But when expressed as absolute amounts, the volumes and surfaces of the cell components showed a substantial increase. The largest changes occurred in total volume of Golgi complex and total surface of endoplasmic reticulum, which both showed a more than two-fold increment. The present investigation provides quantitative histological data showing that pronounced hypocalcemic stimuli may cause an increase in the machinery involved in synthesis and processing of parathyroid hormone.