Little is known about the effect of environmental temperature and caffeine on performance, metabolic, cardiovascular, endocrine, hematological, perceptual, and mood measures during endurance exercise. The purpose of these studies was to compare the effects of ambient temperature on caffeine ergogenicity during endurance cycling exercise. Eleven male cyclists (mean ± SD; age, 25 ± 6 y; mass, 72.6 ± 8.1 kg; V˙O2max, 58.7 ± 2.9 ml·kg-1·min-1) completed four exercise trials in a randomized, double blind, crossover experimental design. After cycling for 90 min (average 65 ± 7 %V˙O2max ) and remaining euhydrated in a warm (33 ± 1°C) or cool (12 ± 1°C) environment, subjects completed a 15-min performance trial (PT; based on total work accumulated). Subjects ingested 3 mg·kg -1 of caffeine (CAF) or placebo (PLA) 60 min prior to and 45 after beginning exercise. Total work (KJ) during the PT was greater in 12°C than 33°C [p < 0.001, η2 = 0.804, confidence interval (CI): 30.51 to 62.30]. Likewise, CAF, versus PLA, increased KJ independent of temperature (p = 0.006, η2= 0.542, CI: 3.60 to 16.86). Compared to PLA, CAF increased KJ in 12°C (p = 0.024, η2 = 0.413, CI: 1.36 to 15.55) but not in 33°C (p = 0.082, η2 = 0.272, CI: -1.84 to 25.84). CAF versus PLA in 12°C and 33°C resulted in few metabolic, cardiovascular, endocrinological, or hematological changes. Further, there were few changes in perception of mood, thirst, thermal sensations, and exertion. CAF significantly reduced feelings of muscle pain in 33°C (P < 0.05), but not 12°C (P > 0.05). However during exercise, Tie increased in the warm environment (peak Tre; 33°C, 39.40 ± 0.45; 12°C, 38.79 ± 0.42°C; P < 0.05) but was not different in CAF versus PLA (P > 0.05). Increased ambient temperature had a detrimental effect on cycling performance in both the CAF and PLA conditions. Environmental heat stress, manifested as whole-body hyperthermia diminished the ergogenic influence of CAF on cycling performance in the warm but not the cool environment. This performance decrement could not be attributed to fluid-electrolyte, cardiovascular, respiratory, metabolic, endocrine, or perceptual effects, but likely was due to the influence of hyperthermia on the central nervous system. These findings suggest that an informed decision to consume caffeine as an ergogenic aid should be predicated on ambient temperature.