Antibodies to morphine produced after its chronic administration can contribute to changes in the central nervous system during opiate abuse. Facilitation of long-term posttetanic potentiation in mossy fibers of the hippocampus in rats with chronic morphine dependence can be reproduced in hippocampal slices from normal animals treated with antibodies to morphine. Incubation of hippocampal slices with ultralow doses of antibodies to morphine had no effect on control rats, but reduced facilitation of long-term potentiation in hippocampal slices from animals with chronic morphine dependence. This confirms the possibility of using ultralow doses of antibodies to morphine for therapeutic correction of mechanisms underlying the formation of drug abuse.