Field experiments were conducted, using ¹⁵N- labelled nitrogen fertilizer, to measure the absolute and relative uptakes of nitrogen by barley from (a) the fertilizer and (b) other sources. Results from 2 years of parallel experiments on several sites are presented, together with previous years' data from one site. The response of unlabelled N uptake to increasing levels of labelled N application seen at Bush was observed occasionally at the other, higher yielding, sites. It was observed more frequently at a site at Balerno (Midlothian), on soil derived from carboniferous till (as at Bush) than at Aberlady, East Lothian, on a raised beach soil. Uptake of unlabelled N generally increased towards the end of the growing season, while uptake of labelled N remained constant or fell. This phenomenon was observed at all sites. Of the various mechanisms possible for the interaction of nitrogen fertilizer with other sources of N, it was concluded that the most likely explanation of the results was a stimulation of crop root growth by the fertilizer, promoting greater exploitation of available N. Significantly higher quantities of N from both sources were taken up by winter barley than by spring barley. In soil under autumn -sown barley, there was less available N in spring than in similar soil newly sown with spring barley. This highlighted the risk of leaching of available N in the soil before it could be taken up by the spring-sown crop. Laboratory measurements of N mineralization indicated a zero-order relationship with time in topsoil. There was also rough agreement with A-values calculated from crop uptake. Interaction between the 2 sources of nitrogen was observed on imperfectly-drained soil at the Bush Estate, Midlothian. There was a general, positive response of unlabelled N uptake to increasing levels of applied, labelled N. There was no evidence of differences in readily-mineralizable N in topsoil due to differential rates of N application over several years. In general, crop yields were greater on the ploughed than on the direct- drilled plots. The exception was in 1979, when the spring was wetter and the summer was drier than in the other years.