Abstract The third-stage larvae of Nippostrongylus brasiliensis have a high rate of oxygen consumption immediately after they are subjected to a temperature rise. When raised to temperatures of 35 °C and below a marked decline in Q O 2 occurs during a period of 3–5 hours after the temperature change. At 38 °C the decline is minimal. Properties of the medium, larval excretion products, and preparative procedures are eliminated as causes of the shape of the Q O 2 -time curve, which is shown to be due specifically to a larval response to temperature change. The nature of this response and its significance in regard to generalizations about the metabolism of infective nematode larvae are discussed.