Two alternative methods for the isolation of functionally intact mitochondria from pig retina were evaluated by means of differential centrifugation. If ATP and bovine serum albumin were present during the isolation procedures mitochondria exhibited ADP sensitive respiration and membrane potentials, comparable to values from other tissues. The mitochondria fraction of both methods differed in purity, being almost free of synaptosomes when a discontinuous Ficoll gradient centrifugation was used.