Rabbit model of fever was produced by means of subcutaneous inoculation of pneumocci. Third ventricular intubation and irrigation were carried out in rabbits. Irrigation fluid was collected during the stage of normal temperature, at the peak of fever and when the temperature was lowered by rhubarb. cAMP was detected with RlA in each of the three portions of the irrigated CSF. The result showed that cAMP level was raised during fever and decreased after rhubarb administration. Irrigation of CSF without medication had no significant influence on cAMP level.