1. The rate of increase in thoracic temperature was strongly dependent on ambient temperature during pre-flight -warm-up in the sphinx moths,Hyles euphorbia andDeilephila elpenor (Figs. 1, 2). 2. The duration of pre-flight warm-up at 19–21 ° C was not correlated with mean body weight (0.3–3.5 g) in 13 species of sphinx moths (Kg. 3). 3. The mean rates of oxygen consumption during uninterrupted free flight forH. euphorbia andD. elpenor were 55 and 60 ml O2/g/hr respectively, regardless of air temperature at 15 and 22 ° C (Table 1, Fig. 4). 4. Thoracic temperature of sphinx moths of 13 species in free flight in the field usually ranged from 38 to 43 ° C, being independent of mean body weight (Fig. 5) but strongly correlated with weight-relative wing area (Fig. 6). 5. These data, which give no indication of regulation of heat production during warm-up or during flight, are in marked contrast with numerous previous publications on endothermy in moths.