Intraperitoneal (i.p.) injection of adult goldfish with 3% thioglycollate (TG) induces an acute peritoneal inflammation connected with a massive influx of inflammatory leukocytes mainly of the head kidney origin. The number of peritoneal exudate cells retrieved on day 2 of the inflammatory response is significantly lower in fish injected with TG and morphine (20 mg/kg b.w.) than in animals injected with TG only. The morphine effect was totally antagonized in fish injected 20 min earlier with naltrexone (1 mg/kg), a well-known blocker of opioid receptors. Light microscopy of the head kidney Epon sections revealed that basophilic granulocytes are common in the control PBS-injected fish and even more frequent in fish injected with morphine only. In sharp contrast basophils are very rare in the head kidneys from animals with the TG-induced peritoneal inflammation, while they are more numerous in fish injected with TG and morphine. Supposedly, the basophilic granulocytes might be involved in the inhibitory effects of morphine on the acute inflammation in goldfish. An involvement of the head kidney in the morphine modulation of the inflammatory response is strongly supported by the detection of opioid receptors in the head kidney cells suspension.