Turn‐over of human alveolar bone removed either in the day or in the night

Human Alveolar bone was removed in the same individual during bilateral mandibular first premolar extraction, the first one between 1000 and 1230, and the second one between 2200 and 0030 , or vice versa. The statistical analysis (Student t‐test for matched pairs) shows : 1. the mitotic index was higher in the specimens taken between 1000 and 1230, while 3H‐thymidine labelled cells index as estimated in a 1‐hour organ culture, was higher in the specimens taken between 2200 and 0030. 2. Alkaline phosphatase activity (a parameter of bone formation) as well as ß‐glucuronidase and acid phenylphosphatase activities (parameters of bone resorption rate), were higher in the specimens taken between 2200 and 0030. These findings reflect the existence of nyctohemeral fluctuations in human bone turn‐over.