The relative importance of skin vs. core temperature for stimulating cold acclimation (CA) was examined by 5 wk of daily 1-h water immersions (20°C) in resting (RG) and exercising (EG) subjects. Rectal temperature fell (0.8°C; P < 0.05) during immersion only in RG. Skin temperature fell (P < 0.05) similarly in both groups. Physiological responses during cold-air exposure (90 min, 5°C) were assessed before and after CA. Body temperatures and metabolic heat production were similar in both groups with no change due to CA. Cardiac output was lower (P < 0.05) in both groups post-CA (10.4 ± 1.2 l/min) than pre-CA (12.2 ± 1.0 l/min), but mean arterial pressure was unchanged (pre-CA 107 ± 2 mmHg, post-CA 101 ± 2 mmHg). The increase in norepinephrine was greater (P < 0.05) post-CA (954 ± 358 pg/ml) compared with pre-CA (1,577 ± 716 pg/ml) for RG, but CA had no effect on the increase in norepinephrine for EG (pre-CA 1,288 ± 438 pg/ml, post-CA 1,074 ± 279 pg/ml). Skin temperature reduction alone may be a sufficient stimulus during CA for increased vasoconstrictor response, but core temperature reduction appears necessary to enhance sympathetic activation during cold exposure.