A new forced-evaporation type skin capsule for measuring local sweat gland activity in humans is described and details of its construction and problems of calibration are presented. The capsule is self-contained, portable, and inexpensive making it well suited for use in arrays to make multiple simultaneous determinations on adjacent skin areas. Sweat is evaporated from the skin surface by a stream of dry nitrogen gas; subsequent changes in capsule relative humidity and temperature are measured by a commercially available thin-film capacitance sensor and a solid-state current regulating device, respectively. These changes are measured within the capsule itself so no external water vapor analyzer is needed. Data acquisition and evaporation rate calculations are handled by an on-line microcomputer.