In doses of 10 mug/kg or more, lysergic acid diethylamide enhanced the fourth potential (DR IV) of the dorsal root potential complex in the cat. Smaller doses of lysergic acid diethylamide did not in themselves alter the DR IV, but revealed an enhancement of the potential by substance P, which by itself had no effect. 2-Bromolysergic acid diethylamide had no action on the dorsal root...