Subsurface cisterns (ssc) in the pineal gland of Meriones unguiculatus have been characterized with transmission electron microscopy, freeze-fracture, morphometry, and three-dimensional reconstruction. Subsurface cisterns (which are observed only in pinealocytes and never in gliocytes) are cisterns of the endoplasmic reticulum (ER) approaching the plasmalemma at a distance of 15-20 nm. Freeze-fracture preparations show that except for some ribosomes at the cytoplasmic face, the cistern membranes as well as the related portions of the plasmalemma are free of special or specifically arranged particles or pores. All ssc have a lumen of 15-20 nm width and underlie 5.6% of the plasmalemma in a single layer; neither collapsed types nor stacks of ssc could be observed. As seen from reconstructions, large ssc are fenestrated and are situated preferentially in regions where the neighboring pinealocyte also bears large ssc. As a consequence, double-sided ssc, which can be observed in sections of (mostly large) ssc, are not a random phenomenon. In regions of the large ssc, adhering junctions are also concentrated. Tubules of 20-nm diameter link ssc with deeper parts of the ER, particularly with the perinuclear cistern and with the tubulo-cisternal network at the trans-side of the Golgi apparatus. Besides ssc proper, a lot of small endplates of ER-tubules are seen close to the plasmalemma. It is suggested that ssc form by the widening and fusing of such ER-tubules and decrease by retraction of ER-tubules from the cell surface.