Central (intracerebroventricular) injections of PGE2, PGF2 alpha caused dose dependent increases in oxygen consumption (VO2) and colonic temperature in conscious rats. The effects of combined injection of maximal doses of CRF and PGE2 were additive, whereas PGF2 alpha and CRF were not. A CRF receptor antagonist, (alpha helical CRF 9-41, 25 micrograms icv) markedly inhibited the effects of PGF2 alpha on VO2 and temperature, but did not affect the actions of PGE2. These data indicate that PGF2 alpha and PGE2 stimulate thermogenesis by two different mechanisms, the former depending on CRF release. PGF2 alpha may be involved in the thermogenic actions of interleukin-1 beta which is also a potent thermogenic agent acting via CRF.