Stereologic studies were performed on rat pinealocytes. In the morphometric analysis the following parameters were taken into consideration: the volume density of the outer and inner membranes, surface areas of the outer and inner membranes, inner membrane to outer membrane surface area ratio, volumes of the outer and inner compartments, and partition coefficients of these compartments. Visual analysis demonstrated three mitochondrial configurational states. Based on the mitochondrial criteria, the existence of three pinealocyte types may be demonstrated. The authors believe that these are the cells in three different functional states.