It is shown that 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H7), a specific inhibitor of protein kinase C, induces neuritogenesis in neuro 2a cells. The percentage of differentiated cells was 9%, 20%, 59% and 85% at 0, 17, 85 and 500 microM H7, respectively. The number of neurites cell increased 2-, 8- and 14-fold over the controls for 17, 85 and 500 microM H7, respectively. These results indicate that protein kinase C plays a key role in the control of differentiation of neural cells and that its specific inhibition may be of basic as well as of practical importance.