One of the most important functions of biological rhythms is to assure that behavioral and physiological functions are timed optimally in with respect to each other and to the environmental cycles. In mammals, this temporal regulation is driven by a circadian timing system which comprises a circadian pacemaker located in the retinorecipient hypothalamic suprachiasmatic nucleus (SCN), along with a complex network of efferent pathways and target tissues. In this study we investigated the existence of circadian variation in the immunohistochemical expression of glial fibrillary acidic protein (GFAP) in the mouse SCN. We used both entrainment and free-running conditions, and examined GFAP expression at different time points according to 24 hour Zeitgeber time (ZT) and circadian time (CT), respectively. Animals were perfused transcardially, and fixed brain sections containing the SCN were processed for GFAP immunoreactivity. GFAP-Ir was quantified by the percentage measure of pixels method for post-hoc statistical analysis of data. The results indicate the existence of a circadian variation on the expression of the GFAP immunoreactivity in the SCN of the animals of both experimental groups furnishing a contribution to the polemic question that has been raised. Further studies, directed especially to the development of new methods for the measurement of the immunoreactivity of this protein, are essential.