The mechanism of toxic action of the chlorinated hydrocarbon insecticides has not been established. Matsumura (1971) and Wooley and Barron (1968) have suggested that DDT may act on the sodium channels of nerve membranes. These investigators have related the instability of the nerve membrane to the toxic symptoms to the intact animal. Of possible 'model' systems, the superior cervical ganglion was selected. The preganglionic and postganglionic transmission and the O2 consumption of the superior cervical ganglion of the rat were measured in vitro following the onset of symptoms after oral dosing. From these parameters the effect of insecticides on axonal and synaptic transmission was evaluated in defining its action in the intact animal. The materials tested included the chlorinated hydrocarbon pesticides - Chlordane, DDT, Lindane, and Toxaphene; three organophosphorus compounds - DFP, Parathion, and Paraxon; and the alkaloid - Nicotine.