The goal of the present study was to identify structural correlates to an observed variability in the response of ventricular muscle action potential configuration to ryanodine. In a previous study, rabbit ventricular trabeculae were shown to display either a shortened action potential (Group I) or lengthened action potential (Group II) in response to the negative inotropic drug ryanodine. The configuration of control action potentials were also different in these two groups. Action potentials in Group I trabeculae exhibited a prominent early repolarization and low plateau, while those in Group II exhibited a small early repolarization and high plateau. Electron microscope morphometric data showed that there is a significant, positive linear correlation between the number of transverse tubules displaying internal couplings with sarcoplasmic reticulum and the change in action potential duration in the presence of ryanodine. No other differences in internal coupling structure were observed. Although variability in the prominence of M-lines was also apparent between myocytes in both Group I and Group II, there is no correlation between the M-line prominence and ryanodine-induced change in electrophysiological behavior. No other structural variability was noted between Group I and Group II trabeculae. It is suggested that the difference in electrophysiological characteristics in the two groups might be due to a difference in T-tubular density. This hypothesis seems most plausible if there is a differential distribution of ion channels between the surface membrane and T-tubular membrane.