This dissertation consists of two parts; “Part A” and “Part B”. The “Part A” consists of phytochemical studies on three indigenous medicinal plants; Berberis brevissima Jafri, Berberis parkeriana Schneid and Berberis royleana Ahrendt. The selected species of Berberis have been investigated for the first time for their phytochemical constituents. Three new (tirahamine (65), 13-nitrotirahamine (67) and peshawarine (68)) and eighteen known compounds (berberine (69), dehydrocheilanthifoline (70), jatrorrhizine (71), berberrubine (72), 8-oxoberberine (73), columbamine (74), palmatine (75), glutamic acid (76), glutamic acid, methyl ester (77), di-glutamic acid (78), di-glutamic acid, methyl ester (79), di-glutamic acid, di-methyl ester (80), docosanoic acid (81), 23a- homostigmast-5-en-3ß-ol (82), β-sitosterol-3-O-β-D-glucopyranoside (83), nonacosane- 10-ol (84), palmitic acid (85) and linoleic acid (86)) have been isolated for the first time from these selected species. The three new bisbenzylisoquinoline alkaloids (tirahamine, 13-nitrotirahamine and peshawarine) have been isolated from B. brevissima. The spectral studies of the two of the compounds tirahamine and 13-nitrotirahamine were performed at different temperature using different solvents. Acetylation and nitration of tirahamine (65) have also been performed. In this part (Part A) we have described the isolation and structural elucidation of the three new and eighteen reported compounds. The structural characterization of the compounds have been achieved by using UV-Vis, IR, EI-MS, ESI-MS, 1 H NMR, 13 C NMR, DEPTq, DEPT-90, DEPT-135, HMBC, HSQC, COSY, NOESY and NOE techniques. xiIn the “Part B” of the dissertation anticancer, antidiabetic, antimicrobial, antitrypanosomal and nematicidal activities of the fractions and the compounds isolated from B. brevissima, B. parkeriana and B. royleana have been discussed. The anticancer activities were performed against six cancer cell lines i.e. L1210 (Murine lyphocytic leukemia), Colon 38 (Murine colon adrenocarcinoma), CFU-GM (Murine granulocyte macrophage colony forming unit), H-116 (Human colon adrenocarcinoma), H-125 (Human lung adrenocarcinoma) and CEM (Humane leukemic lymphoid). Berberine (69) and jatrorrhizine (71) showed significant activity and selectivity against L1210 and Colon 38 cell lines while palmatine (75) was inactive. The antidiabetic activities were performed against Protein Tyrosine Phosphatase 1B (PTP 1B) which is a negative insulin regulator. Amongst the tested compounds, 8-oxob- berberine (73) was significantly active and showed 29 % of the control. The other isolated compounds i.e berberine (69) (35 %), dehydrocheilanthifoline (70) (38 %), columbamine (74) (33 %) and jatrorrhizine (71) (36 %) also showed good activity while glutamic acid (76) (78 %) was inactive. Antimicrobial activities were carried out against four microbial strains i.e Mycobacterium marinum, sporadic methicillin resistant Staphylococcus aureus (SMRSA), endemic methicillin resistant Staphylococcus aureus (EMRSA) and Escherichia coli while antitrypanosomal against Trypanosoma brucei which is a protozoa belonging to the genus Trypanosoma, causing sleeping sickness. Tsetse fly is responsible for the transferring of this parasitic protozoa from infected human and animals. The Berberis brevissima roots xiifraction C (BBR-C) showed good activity against M. marinum (2.4 % of DMSO (D) control), high activity against SMRSA (0.2 % of D control) and EMRSA (0.2 of % D control) while against E. coli it was inactive. Berberine (69) showed good selectivity and activity against SMRSA and EMRSA (1.1 and 0.5 % of D control). Some of the fractions and compounds have also been tested for % mortality of stage two juvenile of root knot nematodes i.e Meloidogyne javanica. Amongst the various fractions, the Berberis brevissima roots fraction A (BBR-A) exhibited the highest mortality (62.22 %) followed by Berberis parkeriana roots fraction C (BPR-C) 57.22 % and BBR-C (54.00 %). Berberine (69) showed the highest nematicidal potential at 97.3 % of the standard (carbofuran) at the highest conc. (300 μg ml -1 ). Jatrorrhizine (71) ranked second and exhibited 59.50 % mortality followed by berberrubine (72) (49.17 %). Dehydrocheilnthifoline (70) was the least effective, nevertheless, showed good mortality at the highest conc.