The observation that both acids and alkalies cause swelling in excised muscles, and that this swelling is suppressed by the addition of neutral salts, is no new one. The pernicious influence of acids on muscle is referred to as a commonplace by Ringer in 1883 (20, 21) and quantitative data of both acid and alkaline swelling are given by Loeb in 1897 (11). Nevertheless, the subject has never yet received systematic investigation at the hands of any one worker. The following record of experiments showing the behaviour of excised muscle in a range of solutions from decinormal alkali to decinormal acid is intended as a contribution towards filling this vacancy. An account of some preliminary experiments on the swelling of excised muscle has already been published (9). The method of work in the present case is precisely the same. The sterno-cutaneous muscle of the frog was used in all experiments, and the temperature was kept constant at 20°C. in a thermostat.