1. 1. Morphine inhibited sciatic-evoked unit impulse activity in the central grey. This effect was antagonized by naloxone. 2. 2. Phentolamine, an alpha-adrenergic blocker, also caused a naloxone-reversible suppression of sciatic evoked unit increases in the central grey. 3. 3. Propranolol, a beta blocker, was without effect when given alone. Propranolol, but not phentolamine, prevented the naloxone-reversible morphine suppression of such evoked activity. 4. 4. These results lead to several testable models of opiate mechanisms. © 1978.