To determine the effect of ambient temperature on the validity of the Polar S410 heart rate (HR) monitor for estimating energy expenditure (EE), EE estimates obtained .during cycling at 30%, 60%, and 80% of maximal oxygen uptake (VO2max) in 25C, 30C, and 35C ambient temperatures, were compared with indirect calorimetry (IC). Three different methods of estimating EE using the monitor were compared: (a) the AHR .method used the actual values for HRmax and VO2max, (b) the PHR method used the .HRmax and VO2max predicted by the OwnIndex software, with resting HR measured in .the ambient temperature for the test, (c) and the PHRTM method used the HRmax and VO2max predicted by the OwnIndex software, with resting HR measured in thermoneutral .(22C) conditions. The traditional method of using individualized heart rate-VO2 relationship to estimate EE was also compared to IC and the Polar S410 methods. In the 25C, 30C, and 35C conditions AHR overestimated [means (SD)] EE from IC by 4% (11), 7% (11), and 9% (8); PHR overestimated EE from IC by 17% (21), 18% (22), and 19% (19); PHRTM overestimated EE from IC by 20% (21), 22% (21), and 24% (19); and .the HR-VO2 method underestimated EE from IC by 8% (12), 4% (12), and 1.4% (11). It .is concluded that the Polar S410 using the actual values for HRmax and VO2max programmed into the Polar S410 (AHR), provides estimates of EE for a group of young women that are within 10% of those from IC. These EE estimates are similar in accuracy .(i.e. 4%-9% difference) to estimates from the traditional HR-VO2 relationship (i.e. 8%-.1.4% difference). Estimates of EE from the Polar S410 based on estimates of VO2max and HRmax (PHR and PHRTM) are unacceptably high (>10%), with errors ranging from 17%-24% difference in estimates of EE. Ambient temperatures above 25C slightly increase the error of EE estimates from the Polar S410 due to the dissociation of HR from .VO2.