Spinal ganglia from 11-day-old chick embryos were incubated in media containing chlorpromazine (CPZ), lidocaine or D-amphetamine and subsequently examined by transmission electron microscopy. CPZ and lidocaine both caused a structural modification of the neuroblasts similar to that induced by the microtubular-disrupting drugs colchicine and vinblastine. That is, there was a partial disappearance of cytoplasmic microtubules with a concomitant increase in the number of microfilaments and the dictyosomes of the Golgi complex changed, with the number of narrow cisternae decreasing and the number of associated vacuoles increasing. Moreover, the dictyosomes were more distinctly separated from each other than in control ganglia. D-Amphetamine did not give rise to any clear changes in either the microtubular system or the Golgi complex and, furthermore, opposed the colchicine-like effects of CPZ. On the basis of these results it is suggested that the subcellular mechanism of action of CPZ involves the cytoplasmic microtubular system and thus secondarily leads to structural and functional alterations in the Golgi complex, viz., in the case of nerve cells, a disturbance in the production and packaging of material destined for the axon terminals.