United States imports of manufactured products during the period 1947 to 1958 are examined in order to investigate the importance of changes in relative prices, ad valorem tariffs, and domestic production as explanatory variables. A cross section technique using first differences of product data over time is employed. Calculations were made after some stratifications of the data in order to illuminate additional aspects of the problem. Variable time lengths were used in the estimating process in an effort to evaluate the effect of time itself on the parameter estimates and also to determine whether peculiarities exist with respect to the particular years involved. The results were interpreted as reflecting institutional changes that have occurred in United States trade policy during this period.