The membrane constants of guinea-pig papillary muscle have been derived by cable analysis, utilizing a single sucrose gap and micro-electrode recordings. In Na-free 20 mmol/l Sr Tyrode, the longitudinal resistance (r i) was increased by 210% from the control value after the passage of ten action potentials (APs) and by 457% after twenty APs but this increase inr i was reversibly abolished by perfusing with Na-containing solutions.r i was not affected by stimulation in Na-containing 20 mmol/l Sr Tyrode. Concomitant with the increase inr i, the conduction velocity of Sr APs was decreased from the control level of 8.6 to 3.1 cm/s after about twenty evoked Sr APs. The twitches elicited in the Na-free Sr Tyrode were large but the relaxation time was comparatively slow (20–30s), whereas those elicited in the Na-containing Sr Tyrode had a large amplitude and relaxation occurred within a few seconds. It is suggested that the increase inr i is due to junctional decoupling, produced either directly, by an intracellular accumulation of Sr2+, or indirectly by an increase in intracellular H+.