1. Seventy-seven neurons in the cervical enlargement of rats anesthetized with urethan were initially antidromically activated using currents ≤30 μA from the contralateral posterior thalamus. A goal of these experiments was to determine the course of physiologically characterized spinal axons within the diencephalon. Therefore, in 38 cases, additional antidromic mapping was done throughout the mediolateral extent of the diencephalon at multiple anterior- posterior planes. 2. Electrolytic lesions marking the recording sites were recovered for 71 neurons. Thirty-one were located in the superficial dorsal horn (SDH); 39 were in nucleus proprius or the lateral reticulated area of the deep dorsal horn (DDH), and one was in the ventral horn. 3. Eight of 38 (21%) neurons that were tested for more anterior projections could only be antidromically activated with currents ≤ 30 μA from sites in the contralateral posterior thalamus. Such neurons are referred to as spinothalamic tract (STT) neurons. Lesions marking the lowest threshold points for antidromic activation were located in or near the posterior thalamic group (Po). Each of the three neurons that were tested responded to noxious heat stimuli. These findings confirm anatomic studies that have shown that a number of STT axons terminate in Po and suggest that such axons that originate in the cervical enlargement carry nociceptive input from the upper extremity. 4. In 15 cases, electrode penetrations were made systematically throughout much of the contralateral ventrobasal complex (VbC). In 17 cases, penetrations were made throughout the intralaminar nuclei contralaterally, including the central lateral nucleus (CL). 5. Many of the axons (27 of the 38 tested, 71%) that were initially antidromically activated from the contralateral posterior thalamus could also be antidromically activated with low currents (≤ 30 μA) and at increased latencies from sites located anteriorly in the contralateral hypothalamus. Such neurons are referred to as spinothalamic tract/spinohypothalamic tract (STT/SHT) neurons. 6. We attempted to determine if individual axons that were antidromically activated from the contralateral hypothalamus continued across the midline into the ipsilateral hypothalamus. Fifteen of the 18 (83%) neurons that were adequately tested could be antidromically activated with low currents from sites in the ipsilateral hypothalamus. 7. None of the neurons that were antidromically activated from the ipsilateral hypothalamus could be followed anteriorly into the telencephalon. However, in 8 of the 15 neurons that were antidromically activated from the ipsilateral hypothalamus, it was also possible to activate them antidromically with low currents from successively more posterior levels on the ipsilateral side. 8. Thirty-nine neurons were antidromically activated from the contralateral posterior thalamus but were not adequately tested for projections to more anterior levels (STT/U neurons). Twenty-four STT/U neurons were physiologically characterized. The majority (92%) responded preferentially (WDR, n = 10) or exclusively to noxious mechanical stimulation (HT, n = 12). Two neurons responded maximally to innocuous mechanical stimulation (low threshold, LT; n = 2). Thirteen of the 15 (87%) neurons tested responded to noxious heat. Therefore, in many respects, STT/U neurons and STT/SHT neurons were similar. 9. The estimated conduction velocity of all axons between the recording site in the cervical enlargement and the initial lowest threshold point in the contralateral posterior diencephalon was 19.6 ± 0.8 m/s (mean ± SE). The conduction velocity of STT/SHT axons between the contralateral posterior thalamus and the contralateral hypothalamus slowed to 10.2 ± 1.0 m/s. 10. The maximum effective spread of currents of 30 μA was ~400 μm (13 μm/μA). The mean threshold for antidromic activation at the lowest threshold points was ~15 μA, and thus it appears that the effective current spread away from the lowest threshold points was probably <200 μm. 11. The results of the present study using antidromic activation indicate that the axons of many of these neurons follow a complex course. The axons ascend to the contralateral thalamus, continue anteriorly into the hypothalamus, cross the midline to the ipsilateral hypothalamus, turn posteriorly and descend into the ipsilateral thalamus or even midbrain. The majority of these neurons respond preferentially or exclusively to noxious stimuli. Therefore, it appears that these neurons are capable of providing information regarding noxious stimulation of the upper extremity to a large number of nuclei on both sides of the thalamus and hypothalamus.