Human Alveolar bone was removed in the same individual during bilateral mandibular first premolar extraction, the first one between 1000and 1230, and the second one between 2200and 0030,orvice versa.The statistical analysis (Student t‐test for matched pairs) shows : 1. the mitotic index was higher in the specimens taken between 1000and 1230, while3H‐thymidine labelled cells index as estimated in a 1‐hour organ culture, was higher in the specimens taken between 2200and 0030. 2. Alkaline phosphatase activity (a parameter of bone formation) as well asß‐glucuronidase and acid phenylphosphatase activities (parameters of bone resorption rate), were higher in the specimens taken between 2200and 0030. These findings reflect the existence of nyctohemeral fluctuations in human bone turn‐over.