Radiolabelled choline was taken up by tissue slices of the cestode Hymenolepis diminuta by a sodium-dependent and a sodium-independent mechanism. The sodium-dependent uptake was saturable, against a concentration gradient, displayed structural specificity, and was inhibited, in part, by hemicholinium-3. Kinetic analysis of the sodium-dependent choline uptake showed an apparent Kt = 2.0 microM and a Vmax = 0.146 pmol.mg-1 wet weight tissue.min-1, which is consistent with a high-affinity choline uptake (HAChU) mechanism. The rate of uptake or release of choline depended on the magnitude and direction of the sodium gradient, was diminished by high- or low-potassium, but was not chloride or sulphate dependent. A homoexchange mechanism for HAChU was not demonstrated. Evidence was obtained to suggest that HAChU or release of endogenous ACh is regulated by autoreceptors. The choline taken up by the HAChU mechanism was but slowly converted to ACh and other products.