In order to learn the effects of a series of aliphatic omega-aminosulfonic acids with varying numbers of carbon chains, changes in the electroencephalogram (EEG) of adult hen were investigated as the criteria. The drug solution (10 microliters) was locally injected into the telencephalon of a curarized animal under artificial respiration and Wulst EEG was recorded near the administered region. No influence was seen on EEG by the administration of amino acids with 1-3 carbon chains (short-chain structure) at a concentration of 6 X 10(-1)M. Characteristic biphasic spikes developed after the administration of amino acids with 4 or more carbon chains (long-chain structure) at a concentration higher than 2 X 10(-2)M, and their amplitudes gradually increased over 1.5-2.0 mV. Spikes were induced by guanyl compounds of amino acids of long-chain structure even at a concentration of 2 X 10(-3)M. Convulsant drugs also induced spikes when used in doses similar to the spike-inducing omega-aminosulfonic acids. The development of spikes by omega-aminosulfonic acids was antagonized by aminosulfonic acids with a short-chain structure. On the basis of the above-mentioned results, omega-aminosulfonic acids can be classified into excitants and depressants according to their activities in inducing characteristic spikes with high amplitudes. In the development of spikes, the excitants and depressants are antagonistic to each other. The doses of excitants which induce spikes are comparable to those of convulsants and coincide very well with the earlier reported relationship between convulsion development rates of excitants and dosage administered to the cerebral ventricle of mice.