Methods employing various types of capsules for the measurement of sweat volume and for the collection of sample sweat were studied; the filter paper method was found to be the most reliable and easiest to use among the method examined. Individually reproducible response patterns were observed both for sweat volume and sweat chloride content, suggesting a classification of the patterns into four types; large sweat volume with high Cl content, large sweat volume with low Cl content, small sweat volume with high Cl content, and small sweat volume with low Cl content. Determinations on various parts of body revealed that regional variations exist in sweat Cl content. In general, sweat Cl content declines according to the following order: head and neck, middle line on the chest, pectoral region on the chest, forearm (flexor and extensor side) and back of hand. In all 12 subjects (5 males, 3 females and 4 children) sweat Cl was higher in winter than in summer. It usually began to rise in early September, toward high winter levels, and fell toward low summer levels from May on. Sweat Cl content was much higher in all adult males, especially in the winter, than in females and children. This resulted in wider seasonal fluctuations in sweat Cl in men than in women and children. The parallel relation between sweat rate and sweat Cl content was exhibited less markedly in subjects who secrete sweat of dilute concentration, and vice versa. The sweat Cl level is correlated negatively to the degree of vascular responsiveness as measured by a vascular reaction test to cold. Findings support the idea that Cl excretion in sweat is controlled mainly by the systemic effect of climatic conditions through its influence upon salt reabsorption activity of the gland and that the ability of sweat glands to excrete Cl may be closely related to the other thermoregulatory functions.