Levels of prostaglandin (PG) E2 and thromboxane (TX) B2, the stable metabolite of TXA2, were measured by radioimmunoassay in cerebrospinal fluid (CSF) collected from the third ventricle and the cisterna magna of conscious cats. In the absence of fever, PGE2 was usually below the threshold of the assay (0.05-0.37 ng/ml), while TXB2 was measurable in the majority of cases and its concentration was greater in the third ventricle (about 0.7 ng/ml) than in the cisterna magna (about 0.2 ng/ml). At either site, TXB2 content rose if any manipulation was required for the collection of samples. PGE2 levels increased to measurable values (max 1.1-1.4 ng/ml) during fever produced by intrathecal or intravenous administration of leucocytic pyrogen. In contrast, TXB2 concentration rose to an average of 2.2-4 ng/ml only when pyrogen (bacterial or leukocytic) was given intrathecally. Moreover, TXB2 elevation, unlike PGE2 elevation, was limited to the uprise phase of the fever. Imidazole, given either intraperitoneally (50 mg/kg) or intrathecally (3 mg), attenuated the pyrogen fever and suppressed any rise in TXB2 levels. At the same time, the drug tended to increase the PGE2 content of the CSF. Evidence was also obtained suggesting that a fraction of PGE2 is bound to CSF protein, and this event may be important to the inactivation of the compound. These findings are consistent with the concept that PGE2 is involved in the sequence of events underlying pyrogen fever. A role for thromboxane A2 in this process remains to be established.