Isolation, structural studies and properties of 37 new substances which are the derivatives of ajmaline, alangiosterol and its derivatives, and markine and related compounds are reported. Among the indigenous medicinal plants covered by the work are: (i) Alangium lamarkii Thawites, (ii) Cassia absus, (iii) Rauwolfia serpentina Benth., R. caffra, R. canescens, R. schueli, R. degeneri, R. densiflora, R. heterophylla, R. indecora, R. sellowii, and R. vomitoria. (Thesis submitted to the University of Karachi).