Prairie voles (Microtus ochrogaster) were held at 5°C or 23°C and fed an energy concentrate or energy dilute diet in a factorial design. With greater energy needs (temperature effects), small intestine tissue mass, mass with contents, and small intestine length increased by an average of 21%. Caecal length, wet tissue mass, and dry tissue mass increased 12%, 17%, and 12%, respectively, while mass of caecal tissues with contents increased 38%. Reduced energy availability (increased food fiber) accounted for a decrease of 14% in small intestine wet tissue mass, but increases of 11%, 46%, 13%, and 13% in caecal length, mass with contents, wet and dry tissue mass. Large intestine dry tissue mass increased with reduced energy availability, and there was a significant interaction between food and energy effects on wet tissue mass. Dry matter content of prairie vole digesta is high compared to larger herbivores, possibly compensating for a low ratio of gut capacity to metabolic requirement. Changes in organ size are explained in terms of function.