&NA;The effects of 3‐methyl‐5‐isoxazole carboxylic acid (MIC) on postburn plasma volume loss have been studied in the anesthetized dog, and the results demonstrated that, when administered before thermal trauma, MIC significantly reduces the plasma volume loss observed at 1 and 2 hours postburn. The reduction of plasma volume loss observed was less than that observed with nicotinic acid administered preburn. A single 0.2‐mg dose of MIC/kg body weight given 2½ hours after a 15% body surface area third‐degree flame burn significantly reduced the plasma volume loss during the next 4 hours. This effect of MIC was approximately the same as that observed with nicotinic acid under the same conditions. The differences between nicotinic acid and MIC under thse conditions were the size of the dose of drug required (0.2 mg/kg of MIC versus 15 mg/kg of nicotinic acid) and the duration of action of the drug (greater than 3 hours for MIC versus 1 hour for nicotinic acid).