Developmental aspects of the formation of rhodopsin in the chick were investigated. The content of rhodopsin was measured in the retina of the developing chick embryo, newly hatched and adult chickens by both spectral and immunological procedures. Rhodopsin was first detected spectrally at day 18 (stage 43) with the level increasing about 6 fold through hatching. The concentration in the adult retina was about one half that present in the 1 day old chick. By the more sensitive procedure of radioiiranunoassay (RIA), rhodopsin was detected as early as day 12 (stage 37), but was undetectable in the 8 day old embryo. The rhodopsin concentration remained low and relatively constant until day 17 (stage 42), at which point it increased rapidly up to hatching. Retina from the newly-hatched chick had a rhodopsin concentration some 30 fold higher than the 16 day old embryo (stage 41). The results of RIA indicate that the biosynthesis of rhodopsin may significantly precede the morphological appearance of outer segments.