Using a cat tail-flick analgetic testing procedure, enhancement of the antinociceptive action of morphine (0.25 mg/kg s.c.) was detected following pre-treatment (1 hour) with 10 mg/kg s.c. of either amitriptyline or nortriptyline. No analgesia was observed following s.c. administration of either antidepressant or saline alone. Both central (amine re-uptake blockade; anticholinergic) and peripheral (decreased hepatic biotransformation) actions of tricyclic antidepressants may have contributed to augmentation of morphine analgesia. © 1979.