We present new data concerning the problem of two-subband transport in a two-dimensional electron gas. We find that the ratio of the mobilities of electrons in the two subbands depends on the details of the confinement potential, while the ratio of the relaxation times determined from the Shubnikov-de Haas effect does not. In all samples studied the relaxation time determined from the amplitude of quantum oscillations in the magnetoresistance is longer for electrons in the upper subband. In addition, we see evidence for the repulsion of quantum levels and the inhibition of spin-flip transitions between quantum levels. © 1988 The American Physical Society.