1. Fever was produced by intravenous injection of lipopolysaccharide E. coli (LPS; 1 micrograms/kg). Immediately after pyrogen administration, body temperature started increasing until it reached a maximum level (1.7 degrees C) at the 3rd hour of the experiment. The febrile response was accompanied by stimulation of metabolic heat production (maximum by 0.31 W/kg), falls in Te (maximum 7.2 degrees C) and Eres (maximum 0.05 W/kg). 2. Thirty minutes after pyrogen administration, the rabbits were treated intravenously with alpha 1-adrenoceptor antagonist, in the form of a bolus injection (1 ml/kg). All the tested drugs reduced fever from 20% by CRN to 105% by PRA. The antipyretic effect of DOX, BMY, PRA, DHBP was associated with inhibition of metabolism and vasodilation of the ear skin. The rest of the compounds produced antipyresis mainly by depression of metabolism. Apart from PRA and CRN, which inhibited Eres three times, the other compounds did not affect heat exchange from the respiratory tract. 3. One may conclude that the antipyretic effect is a general feature of alpha 1-adrenergic blocking drugs, and is accompanied by inhibition of heat production and/or stimulation of heat loss processes. The antipyretic properties of these blockers might be significant from the clinical point of view.