1. To investigate whether there is a nyctohemeral rhythm in bone turnover, we measured serum bone Gla-protein (osteocalcin, an index of osteoblast activity) concentration every 2h and urinary deoxypyridinoline (a marker of bone collagen resorption) excretion for 8h periods in 10 pubertal girls (aged 10-14 years), 15 premenopausal women (aged 20-49 years) and 17 postmenopausal women (aged 50-75 years). 2. The serum concentration of bone Gla-protein and the urinary excretion of deoxypyridinoline were five times higher in the pubertal girls than in the premenopausal women. The urinary excretion of deoxypyridinoline in the postmenopausal women was twice that in the premenopausal women. 3. There was a nyctohemeral pattern in all age groups with mean night-time increases of 28% (P < 0.001) in the urinary excretion of deoxypyridinoline and of 5% (P < 0.001) in the serum bone Gla-protein concentration. 4. There also were nyctohemeral patterns in the urinary excretion of calcium (P < 0.02), sodium (P < 0.001) and potassium (P < 0.001), with decreases at night. There was a negative correlation between the night-time changes in the urinary excretion of deoxypyridinoline and calcium, especially in adult women (P < 0.01). 5. The serum level of parathyroid hormone increased with age, but this effect was only observed at night (01.00 to 07.00 hours). There was a nyctohemeral rhythm of the serum intact parathyroid hormone level at all ages, with a peak in the afternoon and night. 6. Thus, at night, there is a large increase in bone resorption and a small increase in osteoblastic activity, representing a nyctohemeral rhythm of bone turnover.(ABSTRACT TRUNCATED AT 250 WORDS)