Neurons of the rostral ventromedial medulla (RVM) have been implicated in the modulation of nociceptive transmission. In order to further analyze their role in pain behavior, we studied their activity while eliciting the tail flick reflex with noxious heat. Recording sites were regions in the RVM from which microstimulation (less than or equal to 10 microA, 400 mu sec, 50 Hz continuous pulse trains) inhibited the tail flick reflex. Extracellular unit activity and tail temperature were recorded, stored, and plotted with reference to either the time of tail flick or the time when the stimulating temperature reached 45 degrees C. Neuronal discharges were found to be either increased (on- cells), decreased (off-cells), or unchanged around the time of the tail flick. The decreases in discharge were more closely correlated with the tail flick behavior than with the temperature of the stimulus. These off-cells were located at sites of lowest threshold for tail flick inhibition and tended to be ventral to on-cells. We propose that off- cells must pause if the tail flick is to occur, and that this pausing allows the transmission of nociceptive input through spinal reflex loops.