BACKGROUND The Harderian gland (HG) is a tubulo-alveolar gland found within the ocular orbit of animals which present a nictitating membrane. The Harderian gland is regarded as an extrapineal melatonin producing organ and both, photoperiod and melatonin have been shown to exert an important role in the metabolism and morphological features of such a gland. Our results seem to support the presence of melatonin in the nuclei of the HG cells, although our studies have not definitively proved such presence. METHODS An Immunocytochemical anti-melatonin technique was done over free sections of Bouin fixed material obtained from Syrian hamsters. Some of the sections were embedded in an epoxy resin and studied under electron microscope. RESULTS The presence of positive immunoreaction was observed at the level of the nuclear membranes and in close relation to chromatin. No differences were observed between males and females nor between pinealectomized animals and control ones. CONCLUSIONS Our results suggest the binding of melatonin to the cell nucleus in all cell types of the gland. These observations are in accord with the binding studies performed by Acuña-Castroviejo in purified cell nuclei of rat liver (Acuña-Castroviejo et al., 1994. J. Pineal Res., 16:100-112) and the earlier one by Menéndez-Peláez et al. (1993a,b, J. Pineal Res., 15:59-69; J. Cell Biochem., 53:373-3*2) using the light microscope. Our results seem to support the idea of a nuclear action of melatonin and they agree with the observations of Carlberg and Wiesenberg (1995, J. Pineal Res., 18:171-178) about the activation of some orphan receptors by melatonin.