1. Rats were fed with the elemental diet Vivonex for 1 or 3 months and their jejunal histology was compared with that of an equal number of rats fed on a normal diet. 2. After 1 month of Vivonex feeding a significant reduction in the ratio of crypt height: villus height (CH:VH) was found in the Vivonex-fed rats (n = 4) compared with the control rats (n = 4) (P less than 0.05). 3. After 3 months the CH:VH ratio was also reduced in the Vivonex-fed rats (n = 18) compared with control rats (n = 18) (P less than 0.002). Villus height was significantly increased (P less than 0.002) and crypt height decreased (P less than 0.05). 4. Jejunal protein content, alkaline phosphatase and disaccharidase activity were also determined in 12 control and 12 Vivonex-fed rats from the 3 months study. 5. Alkaline phosphatase activity was increased from a control value of 201 +/- 8 to 243 +/- 15 munits/cm in the Vivonex-fed rats (n = 12) (P less than 0.05) but no significant changes in lactase, sucrase or maltase activites were found. The observed decrease in the CH:VH ratio suggested an improved survival of the mature enterocyte population during elemental diet feeding.