SUMMARY1. The effects of taurine on cardiac inotropic state and relaxation were studied in the isolated rabbit heart using solutions containing three concentrations of calcium: 0.7, 1.8 and 3.0 mmol/l.2. Increases in Ca2+caused increases in inotropic indices, but had no effect on rate constant of relaxation.3. Taurine had a positive inotropic effect at Ca2+= 0.7 mmol/l, an equivocal effect at Ca2+= 1.8 mmol/l and a negative inotropic effect at Ca2+= 3.0 mmol/l.4. Taurine had no effect on the rate constant of relaxation at any Ca2+concentration.5. Taurine modulates the positive inotropic effect of Ca2+but neither taurine nor Ca2+affect the rate constant of relaxation.