The aim of this paper is to present the modes of action of venom as construed by Abū 'Uthmān 'Amr ibn Ba[hdotu]r al-Jā[hdotu]i[zdotu] in his Kitāb al-[Hdotu]ayawān. The toxicological information presented by Jā[hdotu]i[zdotu] is a synthesis of data available in his time, but Jā[hdotu]i[zdotu] complemented these early conceptions by his personal theoretical views. Jā[hdotu]i[zdotu]'s fundamental idea is that venoms act through their specific natures; this idea in a way is reminiscent of present-day theory of enzymes.