A simple method of obtaining subgame-perfect feedback policy rules in a rational expectations environment is applied to the problems of a durablegoods monopolist and of a monopolist facing a dynamic competitive fringe. The resulting equilibrium trajectories illustrate a sufficient condition for potential market power to be advantageous. Numerous examples suggest that, in situations where this sufficient condition is not satisfied, it is possible to find parameterizations of taste and technology such that market power is disadvantageous.