To determine the peripheral mechanisms involved in thermal sweating during the hot summers in July before acclimatization and after acclimatization in September, we evaluated the sweating response of healthy subjects (n=10) to acetylcholine (ACh), a primary neurotransmitter involved in peripheral sudomotor sensitivity. The quantitative sudomotor axon reflex test (QSART) measures sympathetic C fiber function after iontophoresed ACh evokes a measurable reliable sweat response. The QSART, at 2 mA for 5 min with 10% ACh, was applied to determine the directly activated (DIR) and axon reflex-mediated (AXR) sweating responses during ACh iontophoresis. The AXR sweat onset-time by the axon reflex was 1.50±0.32 min and 1.84±0.46 min before acclimatization in July and after acclimatization in September, respectively (p＜0.01). The sweat volume of the AXR(1) [during 5 min 10% iontophoresis] by the axon reflex was 1.45±0.53 mg/cm2 and 0.98±0.24 mg/cm2 before acclimati</SUP>zation in July and after acclimatization in September, respectively (p＜0.001). The sweat volume of the AXR(2) [during 5 min post-iontophoresis] by the axon reflex was 2.06±0.24 mg/cm2 and 1.39±0.32 mg/cm2 before and after acclimatization in July and September, respectively (p＜0.001). The sweat volume of the DIR was 5.88±1.33 mg/cm2 and 4.98±0.94 mg/cm2 before and after acclimatization in July and September, respectively (p＜0.01). These findings suggest that lower peripheral sudomotor responses of the ACh receptors are indicative of a blunted sympathetic nerve response to ACh during exposure to hot summer weather conditions.