8-Hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and RU 24969 have been used to investigate whether 5-HT1A and 5-HT1B receptors are involved in the naloxone-induced jumping behaviour of the chronically morphine-dependent mouse. To control for possible interactions with catecholaminergic systems, the effects of alpha 1- and alpha 2-adrenoceptor antagonists were investigated. 8-OH-DPAT and RU 24969, as well as buspirone, ipsapirone and flesinoxan, were found to suppress jumping. The effects were mimicked by the alpha-adrenoceptor antagonists, idazoxan, WY 26392, yohimbine and rauwolscine. Inhibition of 5-HT synthesis with para-chlorophenylalanine (pCPA) had only minimal effects on withdrawal jumping per se; the attenuating effects of 8-OH-DPAT and RU 24969 were not altered in pCPA-pretreated animals. The effects of RU 24969 were blocked by (-)-pindolol and, stereoselectively, by (-)-SDZ 21-009. (-)-Pindolol neither influenced the action of 8-OH-DPAT nor showed any effect per se. The actions of 8-OH-DPAT and buspirone, but not of RU 24969 and idazoxan, were blocked by the 5-HT1A receptor antagonist, spiroperidol. Similarly, both haloperidol and prazosin prevented the attenuating action of 8-OH-DPAT but did not interfere with the action of RU 24969. We conclude that the actions of 8-OH-DPAT and RU 24969 are mediated by postsynaptic receptors. The 5-HT1B receptor appears to mediate the attenuating action of RU 24969; the exact mechanism of action of 8-OH-DPAT remains open but activation of an alpha 1-adrenoceptor is implicated.