A medical device for controlling and delivering a cardiac therapy, the device comprising: a plurality of electrodes (140, 142, 144, 148, 150, 158, 162, 166) positioned along a first ventricle and a second ventricle of a patient's heart; a signal generator coupled to the plurality of electrodes to deliver pacing pulses to the first ventricle and the second ventricle via the plurality of electrodes; a sensing module coupled to the plurality of electrodes to sense cardiac signals; and a controller coupled to the signal generator and the sensing module and configured to: select a first pacing site along the first ventricle of a patient's heart from the plurality of electrodes; select a second pacing site along the first ventricle from the plurality of electrodes; and control the signal generator to deliver a pacing therapy comprising periods of a first ventricular pacing mode and periods of a second ventricular pacing mode by pacing the first ventricle at the first pacing site during the periods of the first ventricular pacing mode and at the second pacing site during the periods of the second ventricular pacing mode; characterized in that the controller is further configured to: determine a first activation time at each of a plurality of sites along the first ventricle in response to pacing pulses being delivered to a second ventricle; select the first pacing site in response to the determined first activation times; determine a second activation time at each of the plurality of sites along the first ventricle in response to no pacing pulses being delivered to the second ventricle; select the second pacing site in response to the determined second activation times; and select the first pacing site and the second pacing site as sites associated with respective first and second activation times that are longer than another of the determined first activation times and second activation times, respectively. a plurality of electrodes (140, 142, 144, 148, 150, 158, 162, 166) positioned along a first ventricle and a second ventricle of a patient's heart; a signal generator coupled to the plurality of electrodes to deliver pacing pulses to the first ventricle and the second ventricle via the plurality of electrodes; a sensing module coupled to the plurality of electrodes to sense cardiac signals; and a controller coupled to the signal generator and the sensing module and configured to: select a first pacing site along the first ventricle of a patient's heart from the plurality of electrodes; select a second pacing site along the first ventricle from the plurality of electrodes; and control the signal generator to deliver a pacing therapy comprising periods of a first ventricular pacing mode and periods of a second ventricular pacing mode by pacing the first ventricle at the first pacing site during the periods of the first ventricular pacing mode and at the second pacing site during the periods of the second ventricular pacing mode; characterized in that the controller is further configured to: select a first pacing site along the first ventricle of a patient's heart from the plurality of electrodes; select a second pacing site along the first ventricle from the plurality of electrodes; and control the signal generator to deliver a pacing therapy comprising periods of a first ventricular pacing mode and periods of a second ventricular pacing mode by pacing the first ventricle at the first pacing site during the periods of the first ventricular pacing mode and at the second pacing site during the periods of the second ventricular pacing mode; characterized in that the controller is further configured to: determine a first activation time at each of a plurality of sites along the first ventricle in response to pacing pulses being delivered to a second ventricle; select the first pacing site in response to the determined first activation times; determine a second activation time at each of the plurality of sites along the first ventricle in response to no pacing pulses being delivered to the second ventricle; select the second pacing site in response to the determined second activation times; and select the first pacing site and the second pacing site as sites associated with respective first and second activation times that are longer than another of the determined first activation times and second activation times, respectively.