The importance of the spinothalamic tract in pain transmission makes it an attractive candidate for study with respect to the effects of antinociceptive compounds. We have been interested in the analgesic actions of opioids and noradrenergic agents at the spinal level and have investigated the effects of these agents on extracellularly recorded nociceptive dorsal horn neurons in the rat. Spinothalamic tract cells were identified by antidromic activation from the somatosensory thalamus. Morphine was administered by bathing the spinal cord in an artificial cerebrospinal fluid solution which contained a known concentration of drug. We observed a dose-related inhibition, naloxone-reversible in some cases, of activity produced by spinally administered morphine in identified rat spinothalamic tract cells and dorsal horn nociceptive neurons. Morphine had no effect on stimulus-evoked responses of low threshold dorsal horn neurons.