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The Oldman River Basin (OMRB), located in southern Alberta (Canada), with an area of 28,200 km(2), is mainly forested in its western part and is used for intensive agriculture in its eastern part. The objective of this paper is to estimate the nitrogen (N) budget for the Oldman River Basin as a whole and its sub-basins, and to discuss differences in the N budget between various sub-basins. Better knowledge of the N budget in this watershed may be also utilized for understanding N dynamics in similar watersheds within semi-arid climatic regions. The model used is a mass-balance spreadsheet model that takes into account N inputs and N export through surface water. During the last 120 years, anthropogenic N inputs to the OMRB have increased circa 40 fold. By the end of the 20th century, the OMRB received an annual N input of about 5174 kg N km(-2) yr(-1), whereas only about 25 kg N km(-2) yr(-1) were exported via riverine flow. For the sub-basins, annual N inputs ranged from 2516 to 19011 kg N km(-2) yr(-1), and annual N export via riverine flows varied between 6 and 277 kg N km(-2) yr(-1). Over 85% of total N inputs to the OMRB are due to anthropogenic activities, including manure (55%), synthetic fertilizer (27%), and N fixation on agricultural lands (4%). Sewage accounted for less than 1%, and N inputs from atmospheric deposition and fixation in forests represented 6 and 8% respectively. Despite increasing anthropogenic N inputs, N export with riverine flow currently accounts for only 1% of the inputs, indicating that most of the N inputs are currently retained in the OMRB or are re-emitted into the atmosphere.