Ultraviolet spectrophotometric analysis has been applied to the determination of nitrate nitrogen over the 0·5 to 10 p.p.m. range in soil solutions and perfusates in the absence of organic matter, and over the 5 to 50 p.p.m. range in its presence. If necessary, a 4-fold increase in ultimate sensitivity can easily be obtained. Sulphamic acid can be used for the destruction of nitrite which would otherwise interfere, and a short procedure has been suggested for the reduction of severe interference present in extracts from highly organic samples such as peat. The more important nitrification inhibitors used in perfusion experiments are well tolerated. Nitrate contents of soil solutions were measured by the method and gave results in good agreement with the nitrate reduction and ammonia distillation method.