1. Cable parameters, component conductances, excitability and membrane potentials in isolated external intercostal fibre bundles at 38 degrees C from normal and myotonic goats were measured in normal and low-chloride Ringer, and in the presence of monocarboxylic aromatic acids that produce myotonic responses in mammalian muscle.2. The mean resting chloride conductance in mumho/cm(2) of myotonic fibres (range 0-147) was significantly less than that of normal fibres (range 376-951). The mean resting potassium conductance was higher in myotonic fibres (range 123-285) than in normal fibres (range 44-132). Potassium conductance increased about 10 mumho/cm(2) per mV increase in absolute resting potential.3. In normal fibres in normal Ringer 3-chloro-2,5,6-trimethylbenzoic acid; 5,6-dihydro-5,5-dimethyl-7-carboxybenz[c]acridine; phenanthrene-9-carboxylic acid; and anthracene-9-carboxylic acid at 10(-5)-10(-4)M decreased membrane conductance without consistently changing diameter or capacitance. In low-chloride Ringer 3-chloro-2,5,6-trimethylbenzoic acid (5 x 10(-5)M) increased potassium conductance in myotonic and normal fibres. It is concluded that these compounds block chloride conductance.4. The carboxylic acids produced myotonia in normal fibres similar to that in untreated myotonic fibres.5. Anthracene-9-carboxylic acid intravenously (8 mg/kg) in normal goats produced acutely a condition resembling myotonia congenita. The carboxylic acids produced no myotonic effects in frog muscle.