The suprachiasmatic nuclei (SCN) in mammals generate and maintain a variety of daily rhythms. In nocturnally active rodents, SCN electrical activity is high during the subjective day (that time of locomotor inactivity) and low during the subjective night. The present experiment examines the relationship between SCN neuronal activity and the expression of overt circadian behavior in the Djungarian hamster (Phodopus sungorus). Wheel-running locomotor activity was measured in 2 groups of hamsters housed in constant dark (DD). Twelve hamsters, selected from ongoing experiments in our laboratory, showed no overt circadian rhythm in locomotor activity. A second group of 9 control animals exhibited rhythmic wheel-running activity in DD. In contrast to control animals, in vitro SCN electrical activity in brain slices prepared from acircadian hamsters showed no circadian variation. These data indicate that in acircadian animals SCN electrical activity properly reflects behavioral patterns.