The oleandrin content of leaves of Nerium oleander L. has been evaluated monthly, over a period of 15 months, by a spectrophotometric method after solvent extraction, chromatographic separation on silica gel preparative layers and column purification. The amount of oleandrin is connected with the cambium meristem rhythm of the stem during different months of the year. It has been shown that the increase of oleandrin content is connected with the cambium reactivation which, in turn, is affected by the weather conditions.