Ultrastructural changes of canine kidneys, in which hyperparathyroidism wa sinduced experimentally by means of parathyroid extract injection, were studied by using electron microscope. The most prominent changes were observed in the proximal convoluted tubules which were thought to be the primary site impaired by excess of parathyroid hormone. The changes in the proximal convoluted tubules were as follows: 1. Tubular lumen was open and the microvilli of brush border were loosely arranged. Cytoplasm of tubular epithelium was often extruded and fell into the tubular lumen.Sometimes fibrillous substance or small spherical bodies, presumably the origin of urinary tract calculi, appeared in the tubular lumen. 2. In the tubular epithelium, vesicles, vacuoles and electron dense bodies including lysosomes were increased in number possibly due to the increase of mucopolysaccharide transport by the effect of the parathyroid hormone. Mitochondria were swollen or vacuolized, and decreased in number. 3. Deposition of electron dense substance was found in the basement membrane which increased in thickness. The alteration was considered probably the first evidence of nephrocalcinosis.