Acute effects of low-dose X-radiation and light illumination on the isolated ganglion cells of Aplysia were analyzed by means of intracellular recording of their synaptic activities. The acetylcholine (ACh)-induced depolarization associated with an increase in membrane conductance was not significantly affected by X-radiation up to 9300 R. Orthodromically activated EPSP's were not markedly altered by X-ray up to 4500 R, but they were slightly enhanced with the higher dose. The threshold firing level of the soma was not affected by a total dose of 13,000 R. Membrane resting potential (MRP) was hyperpolarized 1 to 3 mV during exposure to 1700 to 5000 R but was depolarized 1 to 3 mV during exposure to more than 13,000 R. It was concluded that the central nervous system of Aplysia is much more radioresistant than that of mammals. Light illumination produces either an increase or a decrease in frequency of the spontaneous spike discharge. It was found that the light does not alter the ACh-induced responses of either D- or H-membrane but it depresses the spontaneous EPSP's and IPSP's, indicating the suppression of the presynaptic activities. The threshold firing level of the D-type cells was markedly elevated during illumination. It is suggested that the primary cause of the light-induced change in synaptic activity is the elevation of threshold firing level of the spike at the presynaptic fibers.