Tricyclic antidepressants and selected hydroxylated metabolites were found to inhibit [3H]naltrexone binding in whole rat brain. The drug concentrations required to inhibit binding appeared to be within pharmacological relevant brain concentrations. Carbamazepine and several monoamine oxidase inhibitors (with the exception of clorgyline) were inactive in this regard. The relative potency of the tricyclic antidepressants with regard to inhibitions of opiate binding in brain did not correlate with their clinical efficiency as antidepressants, suggesting that these compounds probably do not exert their antidepressant effects through opioid peptidergic systems in brain. In addition, we found that imipramine (30 mg/kg) had antinociceptive properties, as assessed by the hot plate procedure, which were partially, but not significantly, reversed by naloxone (2 mg/kg). The possibility that opioid receptors may be involved in the analgesic properties of the tricyclic antidepressants has been discussed.