Four systems of traditional medicine have been adopted in Sri Lanka: Ayurveda, Siddha, Unani and Deshiya Chikitsa. The Ayurveda and Deshiya Chikitsa systems use mainly plant and herbal preparations for the treatment of diseases--the former uses about 2000 species, the latter about 500. The plants are used singly or as mixtures. The traditional systems of medicine have a vast literature, mainly in the form of manuscripts. The principle of the Ayurvedic system is to consider the body as a whole, ailments of different organs not being treated separately as in modern medicine. Similarly, Ayurveda takes into account the actions of the drug in its entirety. Research therefore must be carried out in hospitals or biological laboratories and not in chemical laboratories where plant extracts are subject inevitably to chemical reactions. Therefore the chemical approach to identify active principles is a complete deviation from the principles of traditional medicine. Research on plants should be carried out for the further development of traditional systems of medicine and not to their detriment. The threat of extinction of certain species of plants and herbs is stressed, the causes being the destruction of jungles, the greater demand for raw materials for increased manufacture of traditional medicinal preparations, the absence of organised cultivation of medicinal plants, and unscientific harvesting. The compilation of encyclopaedias of plants used in traditional medicine is highly recommended for every country interested in preserving the traditional systems of medicine.