The role fo the Golgi apparatus and the Golgi-endoplasmic reticulum-lysosomal complex (GERL) in the formation of lysosomes in the photosensory cells of regenerating and dark-adapted eyes of the garden snail Helix aspersa was examined with use of acid phosphatase as a marker enzyme. In newly regenerated eyes, lead reaction deposit was restricted to the cisternae of GERL, a few small vesicles and some large secondary lysosomes. Dark-adapted sensory cells, on the other hand, were characterized by a heavy reaction product in the hypertrophied Golgi cisternae and GERL. Primary lysosomes were packaged by GERL cisternae in both the regenerating and the degenerating dark-adapted eyes. In the latter, these lysosomes may have been produced also by the Golgi apparatus. No reaction product was found in the rough endoplasmic reticulum, microvilli, or large aggregations of 800 A photic vesicles in either kind of eye. In this study the lytic activity in differentiating sensory cells was significantly lower than that in degenerating cells suggesting that the increase in lysosomal activity in the latter was due to the absence of light. The effects of long dark-adaption appear to be: 1. decrease in the production of photic vesicles; 2. increase in the formation of lysosomes; and 3. breakdown of photic vesicles by lysosomal activity.