This paper is a summary of studies over the past few years that pertain to animal neurotoxins. These toxins are found throughout the animal kingdom. Homologies exist in the structures of these poisons within classes and point to conservation of active sites throughout evolution. In the case of the peptides, invariant amino acids may be involved in the active site, be essential for maintaining the shape and conformation of the molecule or serve as a fulcrum for folding of the peptide chain after synthesis. At the nuclear or DNA-level, a constant base sequence may regulate gene operation so that only a specific amino acid is coded. Physiologically, and with ultrastructural and biochemical correlation, the predominant mode of action of neurotoxins relate to one or the other of the major activities of the excitable cell,--on conductile activity affecting Na+ or K+ permeabilities, on output or secretory activities affecting the release of neurotransmitter or on in put generator activities affecting the receptor molecules for transmitter themselves. The future of these animal neurotoxins in neurobiological research is secure. The elucidation of molecular mechanisms, by which these various physiological activities of excitable tissue are expressed, will surely involve one or more of these fascinating, naturally-occurring compounds.