A high-performance thin-layer chromatographic method with densitometric detection has been used to determine the convallatoxine content of extracts from the various parts (flowers, leaves, and underground parts such as the rhizomes and buds on the rhizomes) of the plant. Plant extracts were separated on thin layers of silica gel Si 60F254 by multiple gradient development. The convallatoxine content was determined by densitometry and the results were evaluated statistically.