A total of 166 neurons in parafascicular nucleus (PF) were studied, 85 from intact animals, 72 following dorsal spinal cord transection (D.Sp.C.X.), and 9 following complete transection of the spinal cord. Two patterns of nociceptive responses were identified following noxious stimulation and these responses were classified as 'nociceptive-on' and 'nociceptive-off' neurons, respectively. The effects of stimulating the substantia nigra (SNS) on the spontaneous and on the nociceptive evoked discharges were observed and compared in intact, D.Sp.C.X. and completely transected spinal cord rats. The results show that SNS significantly suppresses both the spontaneous and the nociceptive evoked discharges elicited by peroneal nerve stimulation. With an intact spinal cord, SNS suppressed both the spontaneous [-37 +/- 3.2% (P less than 0.05)] and the nociceptive evoked discharges [-52.8 +/- 2.8% (P less than 0.01)] of the 'nociceptive-on' cells respectively, while in the 'nociceptive-off' cells the same stimulation elicited an even more prominent suppression upon both discharges (-47.7 +/- 5.4%, P less than 0.01 and -64.9 +/- 5.0%, P less than 0.01), respectively. After D.Sp.C.X., the suppressive effects on the 'nociceptive-on' cells following SNS were diminished (-28.1 +/- 3.5% and -36.9 +/- 2.6%, respectively) but not abolished, while in the 'nociceptive-off' cells, the inhibitory effects on SNS were unchanged. In addition, the suppressive effects of SNS on the spontaneous activity of PF neurons in cases with completely cut spinal cords remains unchanged. These results suggest that SNS modulates the spontaneous and the noxious evoked responses of the PF neurons by way of supraspinal connections besides the previously described descending projecting pathways.