: To investigate the role of the neuronal growth-associated protein GAP-43 (neuromodulin, B-50, F1, P-57) in neurotransmitter release, we transfected PC12 cells with a recombinant expression vector coding for antisense human GAP-43 cRNA. Two stable transfectants, designated AS1 and AS2, were selected that had integrated the recombinant sequence and expressed antisense GAP-43 RNA. Immunoblot analysis of proteins from AS1 and AS2 cells indicated that the level of GAP-43 in these cell lines was reduced. In the presence of extracellular calcium, a depolarizing concentration of K+ (56 mM) evoked dopamine release from control cells, but not from AS1 and AS2 cells. Similarly, the calcium ionophore A23187 evoked dopamine release from control cells, but was ineffective in stimulating dopamine release from AS1 and AS2 cells. The antisense transfectants, as well as the control cells, contained appreciable quantities of dopamine and secretory granules with a normal appearance. Because the expression of antisense GAP-43 RNA in PC12 cells leads to a decrease in GAP-43 expression and to the loss of evoked dopamine release, these results provide evidence of a role for GAP-43 in calcium-dependent neurotransmitter release.