Method and apparatus for thermally stimulating bitumen and other viscous hydrocarbon products from a subterranean formation in which the product is held. A plurality of wells are formed in the layer, which wells extend from the surface into a generally horizontal direction through the productive subterranean layer. The respective wells are spaced vertically apart whereby a thermal stimulating fluid such as steam can be selectively injected into discrete segments of the layer. Such injection will achieve the most effective sweeping action by the stimulating fluid through the productive layer. There results an orderly downward flow of the produced product and a more uniform and efficient distribution of heat in the stimulated formation.