In the normal chick embryo the neural retina adheres firmly and irreversibly to the underlying retinal pigment epithelium (RPE) starting on day 17--18 of development, coincident with maturation of photoreceptor outer segments. Retinas from earlier embryos (days 13--16) became prematurely attached to the RPE within 30 minutes after death, if held at 37 degrees C, even though developed outer segments were not yet present. Lower temperatures, or failure to enucleate the eye, retarded but did not prevent this postmortem, precocious adhesion. The biochemical factors involved in this process were explored by altering the incubation medium; the factors appear different from those that help maintain retinal adhesion in the adult mammalian eye.