AbstractThe effects of the local anaesthetics bupivacaine, lidocaine, mepivacaine and prilocaine on lysosomal membrane stability have been studied on isolated rat liver lysosomes. The lysosomal membranes were stabilized by the local anaesthetics only within a very narrow range of concentration (around 0.001 M). In lower concentrations there was no effect of the substances while in higher concentrations there was a marked labilizing effect on the lysosomal membranes. The phlogistic effect of an injected suspension of lysosomes in the rat paw was increased by mepivacaine in concentrations of 1 % (0.04 M) and higher. The labilizing effects of local anaesthetics on lysosomes were probably due to precipitation of lysosomal membrane proteins by the local anaesthetics.