The dopaminergic neurons in the ventral tegmental area have major projection to the following areas: the nucleus accumbens, the basolateral amygdala, and the medial prefrontal cortex (mPFC). The dopaminergic projection is involved in the emotional states as well as motivational and cognitive processes. In the present study, to examine the 24-h profile of dopamine (DA) release in the mPFC (prelimbic cortex), in vivo microdialysis study was performed in intact male and cycling female rats. Dailysates were automatically collected via a microdialysis probe from the mPFC every 30 min for more than 24 h under the freely moving condition. DA concentrations in dialysates were quantified by high performance liquid chromatography system with electrochemical detection. Although all rats showed an episodic release profile throughout the day, significant diurnal variation was observed in both sexes of rats: the DA release was high from the late night through the wee hours of the morning, but low in the afternoon. Moreover, the amount of DA released in female rats was significantly greater than male rats (p<0.01). These results suggest that inherent sex and/or gonadal steroid enviroment can affect the dopamine release in medial prefrontal cortex in rats. This is the first report showing the 24 h profile of DA release in the mPFC in both sexes of rats. [J Physiol Sci. 2006;56 Suppl:S196]