The mechanisms underlying variations in perspiration rate at the glandular level are still poorly understood. Human eccrine sweat glands were dissected from the back of 12 adults, cannulated, and stimulated in vitro with methacholine (Mch). The maximal sweat rate and pKA for Mch determined from the dose-response curve for each individual were compared with the anatomic dimensions of the isolated secretory tubules. There was significant correlation between Mch sensitivity (pKA) and the size of the sweat gland, sweat rate per gland, sweat rate per unit length of the secretory tubule, and sweat rate per unit glandular volume. The sweat glands from individuals judged to be poor sweaters exhibited smaller size, lower secretory activity both in vivo and in vitro, and decreased Mch sensitivity compared with glands from physically fit individuals. We conclude that the increased Mch sensitivity and glandular hypertrophy are the two important features of functionally active sweat glands and infer that these parameters could improve as a result of acclimatization to physical exercise and/or heat.