We propose a physical mechanism to explain the origin of the intense burst of massive-star formation seen in physically colliding/ merging, field, spiral galaxies. We consider the detailed evolution of a realistic, two-component interstellar medium (consisting of H2 and Hi) within each galaxy. Also note that, in a typical spiral galaxy - like our Galaxy, the Giant Molecular Clouds (GMCs) are in a near-virial equilibrium and form the current sites of massive-star formation, but the star formation rate is low. We show that this star formation rate is greatly increased following a collision between galaxies. During a collision between two field spiral galaxies, the Hi clouds from the two galaxies undergo collisions at a relative velocity of ∼ 300 km · s−1. However, the GMCs, with their lower volume filling factor, do not collide. The collisions among the Hi clouds lead to the formation of hot, high-pressure remnant gas. The overpressure due to this hot gas causes a radiative shock compression of the outer layers of pre-existing GMCs in the overlapping wedge region. This triggers a burst of massive-star formation in the initially barely stable GMCs. For details, see Jog and Solomon (1990).