How electro-convulsive therapy (E.C.T.) acts, what is the chain of cause and effect from the electrical application to the clinical improvement, are questions whose answers are as uncertain as ever, in spite of the importance of such knowledge to psychiatry and in spite of the researches of nearly twenty years. The experiments reported in this paper are an attempt to elucidate the links of one possible chain. What are the possibilities? I shall review them briefly, restricting the review to the physiological and biochemical aspects and omitting any reference to the psychological or psycho-analytical.