The positive correlation between per capita income and cross-country price levels is called the "Penn-Balassa-Samuelson effect." The most influential explanation of this effect centers around sectoral output productivities as the determinant of the relative price of nontraded goods. The interaction between the change in relative prices and the change in per capita income, the dynamic PBS effect, is less well known. This paper extends the Turnovsky and Sen (1995) model of a small open economy by adding external economies into the production function. The model's dynamics accord well with several features of the empirical data on the dynamic PBS effect.