In three collie dogs with a congenital pit of the optic nerve and associated serous macular detachment, no connection between the subarachnoid space and the pit or subretinal fluid could be demonstrated with subarachnoid injection of India ink or with serial sections of the eyes. A direct communication between the vitreous, the pit, and the subretinal space could be demonstrated, however, in all three dogs. Flow of India ink particles from the vitreous to the pit and into the subretinal space was demonstrated. Indirect evidence to support the above mechanism in humans includes the biomicroscopic observation of channels coursing from the pit toward the subretinal space and the presence of posterior vitreous detachment or liquefaction of the vitreous in patients with optic pits and macular detachment.