We tested the hypothesis that menthol application would reduce the magnitude and initiation of sweating via excitation of cold-sensitive afferent pathways and concurrently via a cross-inhibition of heat loss pathways in acclimatized (swimmers, SW) and non acclimatized (control, CON) subjects in cool water. It was expected this effect to be exaggerated in SW subjects. Eight SW and eight CON subjects cycled at 60% of their VO(2)max, as long as to reach 38 degrees C in rectal temperature (Tre), without or with (4.6 g per 100 ml of water) all-body application of menthol sediment. Heart rate (HR), Tre, sweating rate (SwR), the proximal-distal skin temperature gradient (TSk(f-f)), and oxygen consumption (VO(2)) were measured continuously. VO(2) and HR were similar between groups and conditions. Menthol increased TSk(f-f), Tre threshold for SwR [+0.32 (0.01) degrees C] and Tre gain, while menthol reduced exercise time by 8.1 (4.1) min. SW group showed higher changes in Tre threshold for SwR [+0.50 (0.01) degrees C for SW vs. +0.13 (0.03) degrees C for CON], higher Tre gain, lower time for Tre increase and shorter exercise time [-10.7 (7) min for SW vs. -4.9 (4) min for CON] in menthol condition. Upon exercise initiation, previously applied menthol on the skin seems to induce vasoconstriction, results in a delayed sweating, which in turn affects the rectal temperature. Acclimatized subjects showed higher delay in SwR and earlier rise in Tre, which most probably is due to the inter-group differences in cold receptors activity.