Jejunal perfusion in the rat with Ringer solution containing 10 mmol/l taurocholate removes considerable quantities of protein and brush border membrane enzymes from the intestinal epithelium. The duration of the experiments was 7.5 h. One group of animals was given 200 micrograms cycloheximide per 100 g body weight intramuscularly 1 h before start of the perfusion. Serial estimations of protein and of four brush border membrane enzymes (alanine aminopeptidase, alkaline phosphatase, gamma-glutamyl transferase, and enteropeptidase) were done in the perfusate. The results provide evidence that during the experiments an increasing proportion of the enzymes stems from de novo synthesis. This is consistent with the concept that after loss of 10-30 per cent of enzyme the molecules are replaced by newly synthesized material, provided that the energy metabolism of the mucosa cells remains intact.