Male rats were undernourished from birth to 3 weeks of age and then allowed a period of unrestricted access to food. These animals were subjected to a second period of undernutrition from 9 to 12 weeks. A group of rats undernourished from 9 to 12 weeks only, and a group well-fed throughout life were used for comparison. Six rats from each group were anaesthetised by intraperitoneal injection with sodium pentobarbitone and killed by perfusion with glutaraldehyde at 12 weeks of age and the small intestine removed and its length recorded. Resin-embedded sections 1 micron thick were cut in the transverse plane from proximal and distal regions of each intestine. Point and intersection counting was performed to estimate villous surface area, volume and shape as well as crypt and muscle volume. The body weight of the undernourished rats was significantly (P less than 0.01) reduced compared to controls, with doubly undernourished rats weighing significantly (P less than 0.05) less than singly deprived animals. Rats undernourished once showed larger deficits in villous surface area and volume in distal regions than in proximal ones. However, in rats undernourished during the weaning period and again in later life the proximal part also became affected, while the distal region was not affected further. Therefore, it appears that the distal region of the rat small intestine is more vulnerable to a single period of food restriction during adult life than is the proximal part.