TRW, under NASA sponsorship, has developed a water electrolysis module (WEM) designed to provide 3.6 lb/day of oxygen at a current density of 100 amps/sq ft and at a pressure level of 80 psia. Although designed for aircraft application, the concepts employed in the design of the module make its use in other life support systems possible.One of the ten-cell water electrolysis modules fabricated, and designated as WEM No. 1, has been successfully operated for 7525 hr. These hours consist of 300 hr of parametric, 180 hr of cyclic, and 7045 hr of endurance testing, to date. The endurance test program is being conducted at a current density of 80 amps/sq ft, a temperature of 175 F, and a pressure level of 30 psia.This paper describes the cell and module configurations and the materials of construction selected. Results of the parametric and cyclic test programs are presented and cell performance and servicing and maintenance requirements discussed. The endurance test program of WEM No. 1 is presently continuing.