For an evaluation of the relation to one another of the main cardiac glycosides in the leaves of Digitalis purpurea it is necessary to determine primary– as well as secondary glycosides in these leaves. In the living plant only primary glycosides are present. During the extraction, as part of the assay, a considerable turn-over from primary– into secondary glycosides takes place. By expressing each glycosid as a part of the total amount, and by subsequently expressing the primary glycosides in and adding them up to their secondary form, uncertain enzymatic influences on the results were excluded. This reasonable arithmetic procedure resulted in a standard deviation of less than ten percent for the various pairs of primary– and secondary glycosides. The quantity of both types of glycosides was determined in three different leaf samples of foxglove by means of fluorescence-spectrofotometry on thinlayer chromatography plates.