Serum levels of osteocalcin (S-OC) and lectin-precipitable alkaline phosphatase activity (S-LAP) are sensitive markers of osteoblastic activity. Diurnal variation has been found for S-OC but has not been reported for S-LAP. We measured S-LAP and serum total alkaline phosphatase (S-TAP) in samples drawn every 60 min during a 24-h study period in nine normal subjects and correlated the findings with the diurnal variation in S-OC. A significant (p less than 0.05) diurnal variation in S-LAP characterized by peaks at 1430 hours and 2330 hours and nadir at 0630 hours was found. Peak levels were 30% higher than nadir level (p less than 0.05). S-TAP also varied significantly (p less than 0.05) with nadir at 0630 hours, showing a difference of 23% between peak and nadir levels (p less than 0.05). Significant cross-correlation was found between S-OC and S-LAP and S-TAP when these lagged 4 h after S-OC: r = 0.51 (p less than 0.02) and r = 0.65 (p less than 0.003), respectively. In other words, changes in S-LAP and S-TAP preceded changes in S-OC by 4 h. There were no significant cross-correlations between the non-lectin-precipitable fraction of AP and S-OC. In conclusion, S-LAP varies in a diurnal rhythm closely related with the diurnal rhythm of S-OC. The almost similar patterns in the diurnal serum levels of these two osteoblastic products strongly suggest that osteoblastic activity fluctuates rhythmically during the day in humans.