Numerical experiments with a coupled model have been carried out to test the heat-pump hypothesis for ENSO. The hypothesis states that the level of ENSO activity is controlled by the meridional differential surface heating over the Pacific: either an enhanced surface heating over the equatorial region or an enhanced cooling over the subtropical/extratropical ocean may result in a regime with stronger ENSO events. Moreover, ENSO may be a mechanism that regulates the long-term stability of the coupled equatorial ocean-atmosphere system. The results from the numerical experiments are shown to be consistent with this hypothesis. Specifically, it is found that a stronger tropical heating or a stronger subtropical/extratropical cooling tends to increase the contrast between the tropical western Pacific warm-pool SST and the temperature of the equatorial thermocline water and thereby destabilize the coupled equatorial ocean-atmosphere system. In response, stronger ENSO sets in, which transports more heat downward and poleward, cooling the warm-pool SST and warming the equatorial thermocline water. In the presence of ENSO, the difference between the warm-pool SST and the temperature of the equatorial undercurrent is found to be insensitive to changes in external forcing.