Harrison, Rutherford and Tarr (1997) use a multiregional Computable General Equilibrium (CGE) model with a CES multistage demand system, imperfect competition, increasing returns to scale (IRS), and two endogenous price elasticities of demand perceived by a firm in each national market, in order to quantify the reforms of the Uruguay Round, when firms compete in a quantity setting oligopoly with constant conjectures. This paper argues that the derivation of the price markups is based on two incorrect assumptions, which might affect their empirical results, especially on output and welfare.