Secondary metabolites are defined as a group of natural compounds that are not directly involved in growth, development or reproduction of organisms. The term 'secondary' in the context of metabolic pathways, was introduced in 1891. The functions of secondary metabolites are related to their prevalent potent biomolecular activity acquired in evolution process involving pest and pathogen defense, UV-B-sunscreens etc. The classification of secondary metabolites taking chemical structure and metabolic pathway into consideration could provide important clues to activities of metabolites which lead to interpretation of function acquisition mechanisms of secondary metabolites in evolutional process. In the present study, we examined whether or not classification of alkaloids by ring skeletons can be related to metabolic pathways.