A nondefibrillating and nonfibrillation-inducing energy is delivered at a first internal thoracic location. A first resulting electrical signal is detected at a second internal thoracic location in or near a target region of a heart. A first defibrillation threshold is estimated using the nondefibrillating and nonfibrillation-inducing energy, the first resulting electrical signal, and a target electric field strength. The defibrillation threshold represents an energy that when delivered at the first internal thoracic location creates an electric field strength in the target region of the heart that meets or exceeds the target electric field strength.