In our continuing search for biologically active natural products from Madagascar as part of an International Cooperative Biodiversity Group (ICBG) program, a total of four antiproliferative extracts were studied, leading to the isolation of twelve novel compounds with antiproliferative activity against the A2780 human ovarian cancer line, and one extract with antimalarial activities was studied, which led to the isolation of five new natural products with antimalarial activities against the Dd2 and HB3 malarial parasites. The plants and their metabolites are discussed in the following order: one new xanthone and two known guttiferones from Symphonia tanalensis Jum. & H. Perrier (Clusiaceae); four new diphenyl propanes and one new cyclohepta-dibenzofuran skeleton from Bussea sakalava (Fabaceae); four new cardiac glycosides from Leptadenia madagascariensis Decne. (Apocynaceae); two new and four known alkaloids from Ambavia gerrardii (Baill.) Le Thomas (Annonaceae); five new sesquiterpene lactones from Polycline proteiformis Humbert (Asteraceae). The structures of all compounds were determined by analysis of their mass spectrometric, 1D and 2D NMR, UV and IR spectroscopic and optical rotation data. Other than structure elucidation, this dissertation also involve bioactivity evaluation of all the isolates, synthesis of two interesting alkaloids, as well as a proposal for the possible biosynthetic pathway of the new cyclohepta-dibenzofuran skeleton. / Ph. D.