Plants play a vital role for the existence of life in the Universe. They not only provide food to animals and humans, but also synthesize chemicals which are used for their own defense as well as being used by man as pharmaceuticals, agrochemicals etc. God has gifted Pakistan with enormous varieties of medicinal plants. Out of these plants only 20 % have been studied for their therapeutic potential. Therefore, there is a need of coordinated and well organized efforts in the area of chemistry of indigenous medicinal plants of Pakistan. The present Ph.D. thesis deals with the phytochemical studies on three indigenous medicinal plants of Pakistan namely Plumeria rubra, Plumeria obtusa and Salsola imbricata. The whole thesis is divided into 6 chapters. Chapter 1 offers the introduction of natural product chemistry and economical and pharmaceutical role of Family Apocynaceae and Chenopodaceae on living organism and human health. Chapter 2 deals with the study of various classes of bioactive secondary metabolites obtained from plants, marine organism and fungi. This chapter provides an overview of secondary metabolites their occurrence in natural sources, their economical role as pesticide, drugs, Page 16 agrochemicals, toxins and pharmaceutical and their biological effects on living organisms. Chapter 3 describes the genus Plumeria, its economical, pharmacological and phytochemical properties. In this chapter the pharmacology of various isolates from the genus Plumeria and biological activities of various species and their extract has been discussed. Almost all the phytochemical isolated up to date from the various species of Plumeria has been assembled year wise in this chapter. Chapter 4 is divided into two parts. Part A deals with the phytochemical studies of Plumeria rubra, including structure elucidation and experimental detail of isolated secondary metabolites from this specie. The methanolic extract of this plant was divided into hexane and ethyl acetate. The ethyl acetate soluble part yielded five new compounds: a flavanone glycoside; rubranonoside (161), a sphingolipid; rubranin (162), an iridoid galactoside; rubridoidside (163), two nor-terpenoids; rubrajaleelol (164) and rubrajaleelic acid (165) together with 1-a-plumieride (87), plumieride p-Z-coumarate (133) and plumieride-p-E-coumarate (134), oleanolic acid (114) and b-sitosterol glucoside (166) on column chromatography. Their structures were assigned by ID and 2D-NMR in combination with EIMS, HR-EIMS and FAB-MS techniques. Structures of the known compounds were determined due to spectroscopic analyses or in comparison with the literature. Among them, 87 and 133 are the reported for the first time from this specie. The structure elucidation of these compounds is in process of publication in peer reviewed journal