This study was carried out in order to investigate the effects of age on the urinary excretion of total and non-dialyzable hydroxyproline (OHPr) in normal subjects. We found that total urinary OHPr was negatively correlated with age but, by means of partial regression analysis, no correlation was found after correction for changes in creatinine clearance; on the contrary, non-dialyzable OHPr showed a statistically significant negative correlation with age (r = -0.56) even when creatinine clearance was held constant (p less than 0.05). A highly significant direct correlation was found between total and non-dialyzable OHPr in the whole group (r = 0.54) and when only premenopausal women and men under 60 years of age were considered (r = 0.51). No correlation was found when postmenopausal women and men more than 60-year-old were taken into account. Our data appear to indicate that also the decrease in osteogenetic activity is responsible for the physiological late involutional bone loss; they also show the importance of hormonal changes in inducing an uncoupling between bone formation and resorption.