The pineal gland transduces photoperiodic informations to the neuroendocrine axis through the nocturnally melatonin secretion. This hormonal message plays a major role in the biological rhythm regulation. By autoradiography, more than 130 melatonin putative targets have been reported in the central nervous system (CNS) and in peripheral tissues. However, cross-species consensus concern only a few of them like the suprachiasmatic nuclei (SCN), the master circadian clock, and the pars tuberalis of the pituitary. Recently, MT1 melatonin receptor cDNA have been cloned in several mammals providing us with new tools to investigate its tissular location at the gene level. In the present study, we report a screening for MT1 mRNA by RT-PCR amplification of numerous tissue mRNA.mRNA were extracted from a large variety of rat tissues. To semi-quantify the melatonin receptor mRNA expression level, each cDNA was amplified concomitantly with both beta-actin and MT1 specific primers.In central and peripheral tissues previously reported to bind melatonin, strong PCR signals were logically observed. More surprisingly, a vast majority of studied tissues express MT1 mRNA and then might be responsive to melatonin.Numerous biological functions express diurnal rhythmicity and internal-synchronization. As, most of them apparently do not receive any out-coming neuronal message from the SCN, endocrine communication was proposed to support biological rhythm synchronization. Our present data strengthen the idea that the nocturnally restricted melatonin secretion could be one internal zeitgeber that putatively distributes the endogenous circadian rhythmicity to all tissues expressing melatonin receptors.