The temperature response curves for net carbon dioxide exchange are described for plants of cultivars (cvs.) Vernal and Moapa alfalfa (Medicago sativa L.) grown at day/night temperatures of 30/25C and 20/15C, an irradiance of 25 nE cm−2 s−1 (400–700 nm), and a day length of 15.5 h. Net carbon dioxide intake (NCI) of the tops decreased with increasing temperature from 20 mg dm−2 h−1 at 10C to 5 mg dm−2 h−1 at 40C. The nature of the NCI-temperature response curve was affected by pretreatment temperature, with NCI being lower at all temperatures except 10C after growth at 20/15C. Photorespiration, which reached its maximum value at a higher temperature (20–30C) than that required for maximum NCI, accounted for 22% of the gross carbon dioxide intake (net carbon dioxide exchange in an oxygen-free atmosphere) at 10C and 55% at 40C. Pretreatment affected the relationship between net carbon dioxide output (NCO) and temperature, with NCO being higher at 10C but lower at 30C after growth at 20/15C as compared to 30/25C.