In order to study age- and sex-related changes in calcium-regulating hormones, serum levels of parathormone (PTH), calcitonin (CT), and vitamin D metabolites were determined in 20 young dogs and 11 old dogs. A highly significant increase in serum levels of PTH was observed in old dogs (1.65 +/- 0.09 ng/ml versus 1.05 +/- 0.09 ng/ml in young animals) (p less than 0.001). Serum levels of 25-(OH)-D were significantly decreased in old dogs (11.24 +/- 2.03 ng/ml compared to 20.59 +/- 1.57 ng/ml in young dogs) (p less than 0.001). No significant differences in serum levels of 1,25-(OH)2-D, 24,25-(OH)2-D, and CT were found between young and old dogs. Serum CT levels were higher in males, especially in old animals. Young females had significantly higher serum levels of 25-(OH)-D than males. It seems that for some reason old dogs are in a relative state of vitamin D deficiency and hyperparathyroidism, which suggests the cause of skeletal alterations related to aging. A similar situation is observed in human geriatric patients.