The effects of the brain-penetrating H2 antagonist zolantidine (ZOL, 3 mg/kg, s.c.) were studied on morphine (MOR, 4 mg/kg, s.c.) antinociception (tail flick test) in the presence and absence of previous restraint stress. Animals were handled for 3 days (to reduce handling stress), restrained for 1 hr or handled on day 4, and tested 24 hrs later. As found previously, restraint enhanced the intensity and duration of MOR antinociception. ZOL potentiated MOR antinociception in handled, non-restrained animals, but inhibited MOR action in restrained animals. In contrast, ZOL had no effects on nociceptive responses in either handled or stressed subjects in the absence of MOR. The data suggest that, in the absence of restraint, brain HA acts at the H2 receptor to inhibit MOR antinociception. In contrast, when an animal has been previously restrained, HA enhances MOR antinociception. Thus, brain HA appears to mediate the restraint-induced potentiation of MOR antinociception. Taken with previous results, the present findings suggest that in the presence of MOR, brain HA can provide bidirectional modulation of nociception. The direction of the modulation seems to depend upon the stress experience of the animal.