Intra-firm trade in intermediates between U.S. multinational parents (MNCs) and their Canadian manufacturing affiliates increased dramatically in the 1984–1995 period (i.e., it roughly doubled). Tariff and transport cost declines were far too small to explain this phenomenon. But we show that the advent of improved logistics management practices, including the ‘just-in-time’ (JIT) production system, can explain much of the growth of intra-firm trade. JIT lowers the inventory carrying cost component of intra-firm trade, and, by 1984, this was more important than tariff and transport costs in many industries. We combine regression analysis with numerous case studies to draw our conclusions.