The influence of thyrotropin releasing hormone (TRH) and its postulated metabolite histidyl-proline diketopiperazine (HPD) on sedation and analgesia produced by delta 9-tetrahydrocannabinol (THC) was investigated in mice. Intracerebral administration of HPD (10 microgram) by itself decreased motor activity, whereas, TRH (10 microgram) was without any effect. THC (10 and 20 mg/kg) given i.p. decreased the motor activity and this effect was antagonised by prior treatment with TRH or HPD. Administration of THC produced analgesia as evidenced by delay in the tail-flick reaction time to a thermal stimulus. Pretreatment with TRH or HPD also antagonized the THC-induced analgesia. THC-induced analgesia was not modified by s.c. pretreatment with naloxone, an opiate antagonist. TRH or HPD pretreatment had no effect on the distribution of total radioactivity representing THC and its metabolites in brain and plasma. These studies indicate that TRH and HPD can antagonize THC-induced sedation and analgesia and that HPD may be an active metabolite of TRH. These effects were observed without altering the distribution of the drug and metabolite in brain and plasma, and therefore their effect may involve their direct actions on the central nervous system. Finally THC induced analgesia does not seem to involve opiate receptors and their involvement in the mechanism by which TRH or HPD antagonize THC-induced analgesia is unlikely.