Caecectomized and sham-operated rats were fed a laboratory chow ad libitum and the effects of caecectomy on the digestibility of the food were studied. Compared with sham-operated controls, caecectomized rats showed a decrease in apparent digestibility of organic matter from 77·8 to 73 ·0%, of crude protein from 83·0 to 79·4%, and of 'carbohydrate' from 74·6 to 69·0%. However, faecal water content increased from 41·6 to 54·8 %. StCr-labelled EDTA was excreted faster in the faeces after caecectomy. The colon partly adapted to the loss of caecal mucosa by increased length and thus mucosal surface area.In a second concurrent experiment the effect of caecectomy on the apparent digestibility of food during food restriction was studied. Six caecectomized rats, comparable in all respects to those used in the first study, were fed the laboratory chow ad libitum for 3 weeks. They were then fed submaintenance amounts of food to achieve body weight losses of 40-50 % and to maintain these low weights for 4 weeks. Finally, they were again fed ad libitum for 3 weeks. During the period of restriction the apparent digestibility of organic matter increased from 72·7 to 75·4 %. This was largely due to the increased apparent digestibility of crude protein which rose from 78·4 to 81·9%. Digestibility coefficients returned to control values immediately upon refeeding ad libitum.