In the present experiment we examined spontaneous endogenous in vitro dopamine release from the corpus striatum of female rats during the morning (09.00–09.30 h) and afternoon (15.00–15.30 h) photoperiod on each day of the estrous cycle. In the morning, the spontaneous dopamine release rates of D-1, D-2 and proestrous female rats were characterized by initial low values which gradually increased (approximately 3-fold) over the 2.5-hour in vitro perifusion. In the afternoon, spontaneous release rates of D-1, D-2 and estrous females gradually declined (approximately 2.5-fold) over the perifusion period. This rhythmic diurnal fluctuation was disrupted in the afternoon of proestrus and morning of estrus when release rate profiles remained stable over the entire perifusion period. These results suggest that changes in spontaneous in vitro dopamine release of corpus striata derived from rats in different phases of the estrous cycle may reflect novel in vivo interactions of both photoperiodic and hormonal c