This paper is the last of a set reporting an investigation of the kinetics of jejunal uptake and inhibitory ability of a series of neutral dipeptides, glycylglycine, L-ananyl-L-alanine, L-valyl-L-valine and L-leucyl-L-leucine, with progressively longer and more lipophilic side chains. The results suggested that at pH 5, uptake of L-alanyl-L-alanine, like that of L-valyl-L-valine and L-leucyl-L-leucine, was the result of two processes, uptake of intact peptide and uptake of free amino acid released extracellularly. On the other hand, uptake of glycylglycine was entirely in the form of intact peptide. In contrast to uptake of L-valyl-L-valine and L-leucyl-L-leucine, the proportion of intact L-alanyl-L-alanine taken up by mediated transport was greatest at the lowest concentration studied and smallest at the highest concentration. Taking the series of results as a whole, whereas the corresponding series of amino acids, glycine, L-alanine, L-valine and L-leucine, showed a progressive increase in apparent affinity for uptake and a decrease in Vmax, we could find no such regular progression with the peptides. The results of work on inhibition of uptake of one dipeptide by another were unexpectedly complex. Examples were the very powerful inhibitory effect of L-valyl-L-valine on uptake of glycylsarcosine, not suggested by the Kt of the former peptide, and the failure of glycylsarcosine to cause complete inhibition of uptake of L-alanyl-L-alanine and L-leucyl-L-leucine, though it could completely inhibit uptake of L-valyl-L-valine.(ABSTRACT TRUNCATED AT 250 WORDS)