Positional and temporal correlates for the development of microvillus membranes and for two of the hydrolytic enzymes they contain have been determined and compared with the ability of enterocytes to transport valine during migration from crypt base to villus tip in jejunal tissue taken from rats maintained on diets containing different amounts of protein. Microvillus elongation and the appearance of both aminopeptidase N (APN) and isomaltase (IM) activities reached maximal rates of expression in enterocytes located 16 +/- 5 micron from the crypt-villus junction. This close positional correlation was not found for the later development of the valine transport function. Feeding rats isoenergetic diets containing 20% instead of 5% protein caused significant increases in both villus height and crypt depth without changing the positional correlations described above. The maximal rates for microvillus elongation and APN and IM appearance were greater and occurred earlier in enterocytes taken from rats fed a high-protein diet. The time of onset and capacity to transport valine were found to be closely correlated for rats maintained on high- and low-protein diets. The ratio of APN to IM activity in fully differentiated enterocytes was either 0.7 or 1.2 depending on whether rats had been fed a low- or high-protein diet. The maximal length of microvillus membranes in fully differentiated enterocytes from rats on a low-protein diet was 1.4 times that found in rats maintained on a high-protein diet. Possible ways in which the microvillus membrane structure of enterocytes, enzyme activity and the ability to transport amino acids might be controlled are discussed. Relative estimates are also made of the probable effects that changes in diet will have on the capacity of the intestine to digest and absorb nutrients.