(+)-Fenfluramine, a 5-hydroxytryptamine (5-HT) releaser and uptake blocker, m-chlorophenylpiperazine (CPP), a 5-HT receptor agonist, and clonidine, an agonist at adrenoceptors, were studied for their ability to modify jumping and wet dog shakes in morphine abstinent rats. (+)-Fenfluramine and CPP blocked jumping with no effect on wet dog shakes whereas the reverse was true for clonidine. The results further show that 5-HT mechanisms are preferentially involved in the expression of jumping in morphine-abstinent rats.