This study was performed in order to determine the effects of upstream mass injection on downstream heat transfer in a laminar non-reacting boundary layer. The study differs from numerous previous investigations in that no similarity assumptions are made. A numerical technique known as the method of integral-matrix analysis is used. This approach is a recent outgrowth of the method of integral relations. The complete coupled set of non-reacting laminar boundary layer equations with discontinuous mass injection was solved for this problem using the integral-matrix technique. The effects of mass injection on heat transfer to both sharp and blunt-nosed isothermal flat plates were studied for a Mach 2 freestream. The amount of injection and the length of the injected region were varied for each body. Heat transfer rates were found to decrease markedly in the injected region. A sharp rise in heat transfer was found immediately downstream of the region of injection followed by an asymptotic approach to the heat transfer rates calculated for the case of no injection. An insulating effect was found to persist for a considerable distance downstream of the injection region. The distance required for this insulating effect to die out was found to depend on the length of the injection region as well as the rate of injection.