The circadian rhythm of cholesterol‐7a‐hydroxylase activity in the normal animal is under the control of the hypothalamo‐adrenal axis. In the genetically blind animal, the enzymatic rhythm observed is under the same control. However, in these animals, the rhythm is not synchronized, but can be demonstrated by the following facts: (a) there is a great variation of cholesterol‐7a‐hydroxylase activity at the two nycthemeral times which correspond to the minimum and the maximum enzymatic activities in the normal animal; (b) in the adrenalectomized animals, cholesterol‐7a‐hydroxylase activity is always low; (c) an excellent linear correlation exists between the enzymatic activity and the corticosterone plasma level. These observations indicate the existence of endogenously controlled biological rhythms.