Effect of electroconvulsive shock on rat brain tubulin content was studied during maturation and aging. The results show that electroconvulsive shock had no effect on soluble tubulin in different brain structures of young animals (22 days) while the same treatment produced a marked decline in adult (95 days) and aged (490-511 days) animals. The same treatment produced inhibition of 3H-leucine incorporation into tubulin and decrease of 3H-colchicine binding in the proteins of synaptosomes isolated from the centricephalic structures of all the ages examined. Tubulin biosynthesis by free polysomes was not diminished to the extent which could explain the decrease of tubulin level found in the soluble or synaptosomal fraction. Thus, our results suggest that changes in soluble tubulin content in response to electroconvulsive shock could be a reflection of changes in equilibrium: tubulin dimers--microtubules--membrane-bound tubulin.