Techniques for selectably providing either constant voltage or constant current stimulation are described. A programming device provides a user interface by which a user selects either constant voltage or constant current stimulation, and selects either a voltage or current amplitude based on the selected stimulation mode. The programming device configures a medical device to provide the selected mode of stimulation at the selected amplitude. For example, when a medical device has constant voltage stimulation circuitry, e.g., circuitry including a voltage source, and the user selects constant current stimulation, the programming device configures the medical device to adjust the voltage amplitude based on a measured impedance to provide substantially constant current amplitude.