Using the histochemical method for the demonstration of NADPH-diaphorase activity, the pineal organ of the frog and rat was investigated in serial sections. A positive NADPH-diaphorase activity was demonstrated in pinealocytes and nerve cells in the pineal organ of the frog, but not in the rat. An intense activity existed in the apical portion of the photosensitive pinealocytes of the frog. Large NADPH-diaphorase positive nerve cells (15-20 microns in diameter) were located within the parenchyma of the pineal organ in the frog. Large NADPH-positive nerve cells were more numerous in the rostral than in the caudal portion of the organ, but the intensely stained cells, counting 25-35 in number, showed almost equal distribution and number in the ventral and the dorsal aspect of the pineal organ. In their staining ability, NADPH-diaphorase positive pineal nerve cells resembled retinal amacrine cells. The results in the pineal organ of the frog are discussed in light of previous morphological findings using the acetylcholinesterase reaction, and with electrophysiological results.