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 < 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 < 0.002). Villus height was significantly increased ( P < 0.002) and crypt height decreased ( P < 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 < 0.05) but no significant changes in lactase, sucrase or maltase activities were found. 6. The observed decrease in the CH:VH ratio suggested an improved survival of the mature enterocyte population during elemental diet feeding.