Seventeen alkaloids have been isolated from the leaves, stem and root barks of Strychnos henningsii collected in Tanzania. They are holstiine, splendoline, 23-hydroxyspermostrychnine, 19-epi-23-hydroxyspermostrychnine, retuline, henningsiine, deshydroxyacetylhenningsiine, O-acetylhenningsiine, 3-hydroxyhenningsiine, henningsiine-N(4)-oxide, 23-hydroxyspermostrychnine-N(4)-oxide, 17,23-dihydroxyspermostrychnine, spermostrychnine, henningsamide, O-acetylhenningsamide, deshydroxyacetylhenningsamide and cyclostrychnine. Among these, the henningsiine and henningsamide derivatives are new alkaloids which possess a (16,17) double bond. Cyclostrychnine is a novel heptacyclic C21 alkaloid which presents some similarities with strychnine and has a C-16 to C-23 bond. Structures were established by spectral means and chemical correlations. © 1991.