In order to follow resetting of the rat circadian pacemaking system by melatonin in vivo, the rhythm in the light-induced c-fos expression was used as a marker of the intrinsic rhythmicity of the suprachiasmatic nucleus (SCN). A single melatonin administration during late day to rats maintained originally under a short photoperiod with 8 h of light per day or under a long photoperiod with 16 h of light per day phase advanced immediately the evening rise in the light-induced SCN c-Fos immunochemistry by about 1.5 h. The data indicate that melatonin administration in vivo may instantaneously reset the intrinsic SCN rhythmicity.