T
he toxicity of alkaloids was sought in relation to the geography of alkaloid-bearing plants. Relative toxicity was inferred from the structure of the compounds, which exhibit regular structure-activity relationships. The toxicity of alkaloids from tropical plants is much greater than those from temperate plants. No significant differences were found between herbaceous and woody plants. The alkaloid content of leaves tends to be a positive function of alkaloid toxicity. The ecogeographical distribution of toxicity is interpreted in terms of plant defense.