The parenchyma of parathyroid (PT) glands of neonatal, growing and adult dogs consisted of dark and light chief cells. In growing and adult dogs, dark chief cells had an electron-dense cytoplasm, prominent rough endoplasmic reticulum (rER) and Golgi apparatus. Based on the secretion cycle proposed by Roth and Capen , they were referred to as active cells. Light chief cells had an electronlucent cytoplasm, inconspicuous rER and Golgi apparatus. They were referred to as inactive cells in resting stages or in stages of transition between active and inactive forms or vice versa. In neonatal dogs, dark chief cells were similar to those in growing and adult dogs and light chief cells had the features of the cells referred to as cells in transitional stages. There were no cells in resting stages. The morphological findings suggest that PT activity in growing dogs was similar to that in adult dogs, whereas in neonatal dogs it was different; this may be related to the animal's developmental status or to changes in serum calcium concentration after birth.