Atmospheric budgets of reduced nitrogen for the major political regions of the British Isles are investigated with a multi-layer atmospheric transport model. The model is validated against measurements of NH3 concentration and is developed to provide atmospheric budgets for defined subdomains of the British Isles. The study shows that NH3 emissions, transport and deposition in a specific region can have profound effects on the composition of the atmosphere over other regions. An important feature is the high export of reduced nitrogen from Republic of Ireland and Northern Ireland to England and Scotland. England is also a net exporter of reduced nitrogen. By contrast, Scotland and Wales are approximately "ammonia neutral" as import of reduced nitrogen roughly balances export. Overall, only 7% of reduced nitrogen in the British Isles is derived from non-British Isles sources (29 kt N yr-1), compared with emissions of 387 kt N yr-1. Most of the reduced nitrogen is deposited in the British Isles (64%), while 36% is exported (148 kt N yr-1). © Springer 2005.