The effects of the naturally occurring amino acid taurine (2-aminoethanesulphonic acid) on isometric force development were investigated using skinned muscle fibre preparations. In atrial and ventricular pig heart muscles, as well as in fibres of slow abdominal extensor muscle of crayfish, an increase of submaximal isometric force was observed in Ca2(+)-activated skinned fibre preparations at physiological concentrations of taurine. The maximal isometric force remained unaffected in all preparations. It is assumed that taurine increases the Ca2+ sensitivity of the force-generating myofilaments in mammalian hearts and crustacean slow skeletal muscle fibres.