Previous studies have shown that papillary muscles from hypertrophied cat right ventricles (RVH) exhibit altered mechanical properties which may be associated with defects in the excitation-contraction coupling process. Since calcium influx [as slow inward current (Isi) during the cardiac action potential is thought to be a major determinant of contractile state, we compared Isi-mediated slow response action potentials ( SRAPs ) in papillary muscles from cats with RVH, induced by chronic pulmonary artery constriction, to SRAPs from sham-operated controls. The results show that 1) when depolarized by elevated extracellular potassium (K+o, 22 mM), RVH muscles became inexcitable (as defined here) significantly faster than control muscles; 2) inexcitable RVH muscles required significantly more isoproterenol than controls to restore slow response activity; 3) at all isoproterenol concentrations tested, SRAPs from RVH muscles were reduced in amplitude and duration compared with controls; 4) SRAPs evoked by long duration stimulus pulses in the absence of isoproterenol were also markedly reduced in RVH; and 5) the relationship between resting potential and K+o was the same in both groups. If the alterations in SRAPs observed in RVH are produced by a smaller Isi, this change may be associated with the diminished inotropic state of cardiac muscle caused to hypertrophy due to pressure overload.