Amino acid discharge from a labelled intracellular acid-soluble (acid-extractable) pool occurs as a negative exponential, which is not affected by pool size or concentration of amino acid in the chase medium. Simple negative exponentials were observed over the range 7-37 degrees C, but the slopes were dependent on temperature. Discharge kinetics do not appear to be mirror images of the kinetics of uptake into the acid-extractable pool. The results lend particular support to the efferent limb of the cyclical perfusion model of amino acid utilisation proposed by Wheatley and Inglis (1980).