The aim of the present study was to optimize the use of serum procollagen type I carboxy-terminal propeptide (PICP) as a possible marker of postmenopaus-al-3761ted changes of bone metabolism. Postmenopausal (n = 20) and healthy fertile young (n = 4) women were studied after informed consent. The postmenopausal women were subdivided in 4 groups: (1) nontreated; (2) treated with estrogen-progestogen replacement therapy; (3) treated with calcitonin, or (4) with kidney or liver diseases. Blood samples were collected at 15-min time intervals for 4, 6 or 8 h. Serum concentration of PICP was measured by radioimmunoassay, in duplicate at two different dilutions. In postmenopausal women mean ± SEM serum PICP levels were slightly but nonsignificantly higher than in fertile women. Serum PICP levels in estrogen-progestogen or calcitonin-treated women were significantly lower than in non-treated postmenopausal women. Episodic changes of circulating PICP level were observed in fertile and postmenopausal women. The pulses of serum PICP levels did not show significant differences among the groups of women studied. The present study showed that the measurement of serum PICP levels is a useful marker for investigating the changes of bone metabolism. In particular, low PICP levels in postmenopausal women under steroid hormone or calcitonin treatment in part reflect the changes of bone turnover. The pulses of serum PICP levels during a time interval suggest that collagen metabolism in women undergoes a rapid turnover