1. Unlike most desert rodents, grasshopper mice, Onychomys torridus, exhibited not reduction in basal metabolic rate from that predicted for mice their size and had high pulmocutaneous water loss.

2. Their thermal neutral zone was 30-35 C.

3. Fecal and urinary water losses were a function of water intake.

4. Food intake increased with water deprivation and the mice could maintain their weight on a beef liver diet containing 72% water.

5. Data from this study suggest that problems of water balance in this species are most severe in winter.