We have studied the postnatal development of dihydronicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) activity in the superior colliculus (SC) and the ventral lateral geniculate nucleus (LGv) of the rat. We describe two different developmental patterns of NADPH-diaphorase activity. The first pattern, observed in the deep layers of the SC, shows a transient activity during the first week which progressively decreases during the following two weeks. The second pattern is observed in the superficial layers of the SC and in the LGv. They become positive during the first week, their NADPH-d activity increases progressively during the second and third weeks, reaching the adult pattern at the fourth week. On the whole, the developmental chronology of the laminar distribution of NADPH-d in the SC displays an inside-out pattern. Our results suggest that NADPH-d activity may play different roles at different stages of the developing nervous system.