Extracellular recordings were performed on electrophysiologically identified achromatic neurons of the photosensitive pineal organ of the frog. The effects of iontophoretically applied aspartate and glutamate as well as one aspartate antagonist ( D-α-aminoadipate) were observed on spontaneous activity and light-evoked responses of pineal neurons. Aspartate and glutamate showed a powerful excitatory effect on achromatic neurons. The effect was dose-related and reversible. D-α-Aminoadipate substantially diminished the excitatory aspartate effect. The changes observed in the ganglion cell responses after application of the drugs are discussed in light of the possible neuronal circuitry of the pineal organ.