Several parameters of excitation-contraction coupling were compared in two types of muscle, using thin strips from the left atria and papillary muscles from the right ventricles of guinea-pigs. (1) The duration of the action potential and twitch is much longer in ventricular than in atrial muscle. (2) Mechanical restitution can usually be described by a monoexponential function in ventricular and biexponential function in atrial muscle. (3) Post-extrasystolic potentiation, when related to the steady state force, is greater in ventricular muscle. (4) When priming with paired-pulse stimulation, mechanical restitution can be studied after the short interval and after the long interval. In atrial muscle, mechanical restitution is very similar after the short and long intervals but in ventricular muscle they are different in size. (5) Ryanodine (10(-6) M) can decrease the steady state force to about 10% of control in atrial but only to about 35% in ventricular muscle. Ryanodine (10(-8) M) causes the slow phase of restitution in atrial muscle to disappear but in ventricular muscle only increases the rate of mechanical restitution. (6) Ca-antagonists (Cd2+ 0.2 mM) can decrease the steady state force to zero in atrial and ventricular muscle. Ca-antagonists, in low concentrations (Cd2+ 0.01 mM), mainly affected the fast phase of mechanical restitution. (7) The recirculation fraction of calcium was about 0.64 in atrial and 0.27 in ventricular muscle. The findings are discussed in the light of known ultrastructural differences between atrial and ventricular myocardium.