A medical device and associated method control the delivery of a cardiac pacing therapy by selecting first and second pacing sites along a first ventricle of a patient's heart and delivering the pacing therapy by pacing the first ventricle using the first pacing site during the periods of a first ventricular pacing mode and using the second pacing site during periods of a second ventricular pacing mode. In one embodiment, the device determines activation times at multiple sites along a ventricle in response to pacing pulses being delivered to the opposite ventricle. A first pacing site is selected in response to the activation time determination. The device delivers the pacing therapy by pacing the first ventricle using the first pacing site during periods of the first ventricular pacing mode.