The work reported here deals with the bio-assay guided isolation of secondary metabolites from Cestrum nocturnum (leaves) and Cestrum diurnum (leaves and berries). First part of the dissertation deals with the biological activities of different parts of the selected plants while second part describes the purification and characterization of the isolated constituents. Shade dried plant material was extracted with methanol. The concentrated methanolic extract was fractionated with n-hexane, chloroform, ethyl acetate and iso-butanol and extractive values determined. All the collected extracts were screened for anti-bacterial, anti-fungal, anti-oxidant and anti-leishmanial activities. The anti-oxidant experiment was also conducted with all the parts. Crude extract of aerial part of C. nocturnum displayed 57 % antioxidant activity while one of its isolated fractions (ethyl acetate) displayed a maximum of 67 % antioxidant activity. Similarly, the crude extract of aerial parts of CD i.e., aerial part without fruit, green fruit, ripe fruit, exhibited 86, 81 and 90 % antioxidant activities respectively. The hexane fraction of CD green fruit showed higher antioxidant potential (87 %) than its crude methanolic extract. Due to our interest in saponins for further studies, they were selectively isolated and processed in the same way for antioxidant activities in crude form from both the plants. The highest antioxidant activity was found with the CD ripe fruit CS extract (90 %) while the least one was with both the aerial parts of the two plants (approx. 80 %). The results obtained with nine microbial strains were moderate. S. typhii was the most resistant strain. On the average gram positive bacteria showed good zone of inhibition with Cestrum diurnum. Gram negative strains showed good results mostly with Cestrum nocturnum. The crude saponins extracts of berries were the most active giving zone of inhibition in the range of 8.5-15mm. Incredible anti-leishmanial activities are being reported for the selected plants. All the plant extracts exhibited hundred percent inhibition of the leishmania parasite. Second part of the thesis deals with the isolation and purification of the secondary metabolites of C. nocturnum and C. diurnum. Hum-V isolated from C. nocturnum was characterised using different spectroscopic techniques as (25R)- spirost-5ene-2,3-diol, [2α, 3β] 3-O-{β-Dglucopyranosyl (1→4)-β-D-glucopyranosyl (1→2)-β-D-glucopyranosyl{(1→4)-β-Dxylopyranosyl] (1→4)-β-D-galactopyranoside). It was reported as nocturnoside A. Another reported compound was also purified and characterized as karatavoiside A, spirost-5-ene-2,3-diol [2α, 3β,25R], 3-O- [β-D-glucopyranosyl(1→2)] {-β-Dxylopyranosyl( 1→3)-β-D-glucopyranosyl(1→4)}-β-D-galactopyranoside. A new compound isolated from the same specie, elucidated as 3-O-β-D-xylopyranoside-olean- 12-en-28-oic acid-28-O-β-arabinopyranosyl-(1-3)-β-D-galactopyranosyl-(1-2)- β-Lglucopyranosyl-( 1-4)- β-L-glgucopyranosyl ester.