A stable sustained rhythmic activity (SRA) occurred in 62 of 192 specimens isolated from the infarcted subendocardium of 48 dogs 24 h after a left descending coronary artery occlusion. Changes in [Na+]0 and/or [Ca2+]0 of the superfusate allowed us to distinguish two types of responses, suggestive of two different mechanisms for SRA. In type 1 responses at constant [Na+]0, the rate of SRA decreased when [Ca2+]0 was increased and increased when [Ca2+]0 was decreased. When [Ca2+]0 was held constant, the rate of SRA was directly related to [Na+]0. In type 2 responses, at constant [Na+]0, the rate of SRA was directly proportional to [Ca2+]0. In contrast, at constant [Ca2+]0, SRA was inversely proportional to [Na+]0. When a constant [Ca2+]0 [Na+]03 ratio was maintained, the type 2 response became indistinguishable from the type 1 response. The combination of lower temperature (36°C) and high initial [Ca2+]0 (2.7 mm) favored the type 2 response (28 of 32 preparations). In contrast, 25 of 30 preparations studied at 39°C and 1.35 mm [Ca2+]0 showed the type 1 response. These results suggest that SRA in the 24-h infarct model can be due to both abnormal automaticity (type 1) or triggered activity (type 2) and that changes in temperature and ionic milieu will largely determine which of the two mechanisms is responsible for SRA. © 1991.