The existence of two separate groups of membrane-enclosed channels in platelets, the open canalicular system (OCS) and the dense tubular system (DTS), has been recognized for several years. The present study has employed ultrastructural observations on normal platelets, cells from a child with the gray platelet syndrome and cytochemical localization of peroxidase in platelets to define the origin of the DTS and point out a specialized membrane complex in platelets formed by interaction of channels from the OCS and DTS. The similarities of membrane complexes in platelets to the organization of membrane systems in muscle is pointed out, and their potential function as the sarcoplasmic reticulum of platelets is discussed.