To study the effect of post-exercise cold water immersion (CWI) on core temperature and melatonin responses, 10 male cyclists completed two evening (~1800hours) cycling trials followed by a 15-min CWI (14°C) or warm water immersion (WWI; 34°C), and were then monitored for 90 min post-immersion. The exercise trial involved 15min at 75% peak power, followed by a 15min time trial. Core (rectal) temperature was not different between the two conditions pre-exercise (~37.4°C), post-exercise (~39°C) or immediately post-immersion (~37.7°C), but was significantly (p<0.05) below pre-exercise levels at 60 and 90min post-immersion in both conditions. Core temperature was significantly lower after CWI than WWI at 30min (36.84±0.24 vs. 37.42±0.40°C, p<0.05) and 90min (36.64±0.24 vs. 36.95±0.43°C, p<0.05) post-immersion. Salivary melatonin levels significantly increased (p<0.05) from post-exercise (~5pM) to 90min post-immersion (~8.3pM), but were not different between conditions. At 30 and 90min post-immersion heart rate was significantly lower (~5-10bpm, p<0.01) after CWI than WWI. These results show that undertaking either CWI or WWI post-exercise in the evening lowers core temperature below baseline for at least 90min; however, the magnitude of decrease is significantly greater following CWI. The usual evening increase in melatonin is unaffected by exercise or post-exercise water immersion undertaken between ~1800 and ~2000hours.