The effect of the organic calcium channel blocker, verapamil, on quinolinic acid (QUIN) neurotoxicity in rat hippocampal cultures was studied. Verapamil and QUIN, both in 100 microns concentration, were added simultaneously to the culture medium. Ultrastructural analysis showed that verapamil was able to reduce typical QUIN-induced tissue damages. Especially, 3 and 7 days after exposure to the two agents, majority of both neurons and postsynaptic dendrites revealed normally appearing morphological features. The results support the suggestion of the important role of calcium entry in the development of QUIN neurotoxicity.