Abstract: Abstract This paper deals with the taxonomical problems of the genus Cynanchum and its allied genera. So far the constituents of 17 species in these genera have been studied. We summarized the constituents, especially the C21 ste-roidal constituents, among which there are 46 C21 steroidal glycosides and 47 steroidal aglycones, of these genera. We classify these aglycones into two basic types of skeletons, i. e. the formal skeleton (skeleton A) and the informal skele-ton (skeletons B, C, and D). Based on this and other scientists' work, we put forth the plausible biogenetic pathway of the Cgi steroidal aglycone. Cynanchum Linn. is a 'difficult taxon' in Asclepiadaceae. E. G. Pobedimova (1953, 1973) , M. Kitagawa (1940, 1959) , K. H. Rechinger (1970) , F. Markgraf (1971) and H. K. Airy Shaw (1973) treated it as several different genera, including Vincetoxicum Wolf. But other taxonomists thought that it is a separate genus. Prof. Tsiang Ying et Li Ping-to (1974, 1977) dealt with Vince-toxicum Wolf as a section of Cynanchum, but they also admitted that their trea-tment must be revised when new evidences of cytology, chemistry and palaeobotany are found.

Key words: Cynanchum