We analyze the Nash equilibria of a one-stage game in which the nature of the strategic variables (prices or quantities) is determined endogenously. Duopolists producing differentiated products simultaneously choose either a quantity to produce or a price to charge. In the absence of exogenous uncertainty, there exist four types of equilibria with differing levels of output: (price, price), (quantity, quantity), (price, quantity), and (quantity, price). The multiplicity of equilibria stems from each firm's indifference between setting price and quantity, given its conjecture about its rival's strategy. But exogenous uncertainty about market demands, which makes firms uncertain about their residual demands, even in equilibrium, gives firms strict preferences between setting price and quantity. As a result, the number of equilibria is reduced. When uncertainty is exogenous, we analyze the effect of the slope of marginal costs, the nature of the demand disturbance, and the curvature of demand on firms' propensities to compete with price or quantity as the strategic variable. These three factors are likely to influence the nature and intensity of oligopolistic competition.