We have examined the effects of 1 alpha-hydroxycolecalciferol (1 alpha) (1.5 micrograms for 5 days) on serum osteocalcin (OC) and other parameters of bone and mineral metabolism in 20 osteoporotic women and 11 age-matched normal women. After 1 alpha administration, a statistically significant (p less than 0.01) increase of serum OC, calcium and phosphate and urinary calcium and hydroxyproline was observed. In contrast, alkaline phosphatase was unchanged. There was no significant difference between normal and osteoporotic women in the changes of any of the parameters we monitored. In particular, OC increased in a comparable way in both groups. Thus, by means of an index that reflects the global activity of the skeleton, we could not find any apparent defect of osteoblastic responsiveness to the drug in osteoporotic subjects.