A non-invasive method of recording cardiac activity was used to examine the impact of chloride level in water on narrow-clawed crayfish. This method permits one to record heart rate without any harm to the animal, and also locates changes in the shape and amplitude parameters of the response, which characterized the crayfish functional state. Altogether, eight levels of chloride (100, 400, 800, 1600, 3200, 6400, 12 800 and 25 600 mgL^-1 NaCl) were evaluated. Already at low levels some crayfish were influenced. A clear reaction was evident starting from 3200 mgL^-1 NaCl. On the contrary, crayfish showed a high tolerance to high chloride levels, and the heart rate and stress index returned to normal within a few minutes or hours after NaCl addition.