Ultrastructural changes in pinealocytes were investigated qualitatively and quantitatively in mice exposed to low temperatures at 0 degree C or 5 degree C for 30 or 60 min. A quantitative estimation of the granulated vesicles of about 100 nm diameter in pinealocytes demonstrated that the number of these vesicles per pinealocyte in mice exposed to cold at 0 degree C or 5 degree C increased remarkably as compared with that of the controls. This finding indicates that cold stimuli may activate the formation of the granulated vesicles in pinealocytes. In mice exposed to cold at 0 degree C for 60 min, granulated vesicles were often found in gourps close to the plasma membrane of the pinealocyte cell bodies or processes, indicating the release of the vesicle content into the extracellular spaces. Additionally, the pinealocytes of the mice exposed to cold at 0 degree C for 60 min were characterized by the frequent occurrence of lysosome-like bodies and filaments. The development of these structures in response to severe cold may represent a depressed functional state of pienalocytes.