A 30 sec swim in water at 30 degrees C reduced the number of abdominal constrictions produced in mice by the intraperitoneal injection of acetic acid. In male mice this reduction in abdominal constrictions induced by swim stress was not affected by prior subcutaneous administration of naloxone hydrochloride. However, in the female mice naloxone hydrochloride administered 5 min, 10 min, or 15 min before the 30 sec swim dose-dependently antagonized the effect of swim stress on the abdominal constriction response to i.p. acetic acid. In view of the possibility that female mice may perceive the stress at different intensity from those of the male, the effects of different swimming durations on female mice were also studied. It was found that a 15 sec swim in water at 30 degrees C was sufficient to induce antinociceptive response to i.p. acetic acid. The antinociceptive effect was greater if the duration of swim was extended to 60 sec. In both instances, prior administration of naloxone dose-dependently reduced the antinociceptive effect induced by swimming. These results suggest sex difference in the involvement of endogenous opioid system in swim stress-induced antinociception in mice.