Despite widespread use of the writhing response to phenylbenzoquinone or acetic acid injection in mice as a screen for analgesic activity, possible mediators of this response remain unidentified. 2. In the present experiments both in vivo and in vitro techniques have been used in an attempt to identify these mediators. 3. Acetic acid-induced writhing was unaffected by pretreatment with mepyramine, methysergide, disodium cromoglycate or cimetidine. 4. Phenylbenzoquinone-induced writhing was partially blocked by pretreatment with mepyramine or other antihistamines. 5. Using guinea-pig isolated ileum and rat fundus preparations as in vitro assay systems, no significant differences were detected in histamine, serotonin, or prostaglandin content of peritoneal fluid from writhing and control mice. 6. Acetylcholine levels were at the limit of detection in these samples, and there was no evidence of kinin release. 7. Thus the mediators involved in the writhing response remain undefined.
