Previous reports have suggested that the rate of de novo cholesterol synthesis in the adult vertebrate retina is extremely slow. We investigated cholesterol biosynthesis in the adult rat retina in vivo, following intravitreal injection of [3H]acetate. HPLC analysis of retinal non-saponifiable lipid extracts revealed co-elution of radioactivity with endogenous cholesterol mass within 4.5 h post-injection. Incorporation of [3H]acetate into cholesterol was markedly reduced by co-injection of known inhibitors of the cholesterol pathway. In contrast to previous results with retinas from other species, no radiolabel or mass corresponded to squalene, except in lipid extracts from retinas treated with NB-598, a squalene epoxidase inhibitor. These results demonstrate, for the first time, the capacity of the adult vertebrate retina to rapidly synthesize cholesterol de novo. © 1993.