The morphological and histoenzymological changes were analyzed in neurons of the bone marrow and spinal ganglia of outbred dogs after they were epidurally administered clonidine in acute (6.4 micrograms/kg) and chronic (15 micrograms/kg) experiments. No pronounced structural-and-metabolic changes in the nervous tissue were revealed after clonidine administration. A certain morphological-and-functional realignment in the spinal nervous cells and ganglia is typical of both experimental animals and controls with a 0.9% sodium chloride solution being administered in the latter group within the chronic experiment. The registered shift did not exceed the limits of biological deviations, which most probably was indicative of the functional reaction in the studied nervous-tissue structures to the epidural injections of drugs.