Abstract Published and unpublished data were used to examine land use changes in the Loch Leven catchment and to estimate annual inputs of N to the loch for five-yearly intervals between 1950 and 1985. Agricultural land (38% grass, 31% arable and 31% rough grazing) was identified as the major source of N entering the loch. The input from this source was estimated to have increased from 171 t N yr −1 in 1950 to 321 t N yr −1 in 1985. This increase was primarily a result of increased usage of N fertiliser. Nitrogen from livestock wastes and that released following ploughing of grassland also made a significant contribution but only that from the former increased during the study period. The greatest increase in fertiliser use was in the quantities applied to grassland but because of the relatively low leaching losses associated with this land use, losses from arable land in all years were higher than those from grassland. Total inputs of N to the loch were estimated to have increased from 208 to 368 t N yr −1 between 1950 and 1985, equivalent to an increase in the average concentration in drainage water from 1.7 to 3.0 mg N l −1. There was approximate agreement between the estimates for later years of the study period and corresponding values derived from field measurements.