This study investigates the behavioral effects of morphine administration and exposure to high ambient pressure in the formalin test. Rats were simultaneously given formalin (0.1 ml, 5%) in a hind paw, and saline or morphine (2.5-10.0 mg/kg) subcutaneously. They were then exposed to ambient pressure of either 1 or 48 bar (compression rate: 3 bar/min; 1 bar is approximately equal to the pressure of 1 atmosphere) in a helium-oxygen atmosphere. The behavior of the animals was monitored for 35 min at stable pressure, starting 25 min after the injections. After morphine, the groups tested at 1 bar showed a dose-dependent reduction in pain-related activities such as licking, biting, clutching and protecting the injected paw but paw-elevation while resting was significantly increased after the highest dose. The 48-bar groups spent almost no time in these behavioral categories but showed an increase in apparently normal motor activity. Paw-jerking appeared to be a more robust response. The number of jerks was not altered by pressurization and was dose-dependently reduced by morphine at both pressures. The results show that hyperbaric exposure alters the response pattern in the formalin test, demonstrate the advantage of evaluating several behavioral criteria in this test and provide tentative evidence against pressure reversal of morphine analgesia.