The effect of nitrogen addition on the activity of rhizosphere bacteria was studied using barley seedlings. Three different nitrogen sources were added to the soil (nitrate, ammonium and ammonium + nitrate) at four different concentrations (0, 100, 300 and 500 mg N kg(-1) soil) and the plants were allowed to grow for 6 weeks. The bacterial activity was estimated by measuring thymidine and leucine incorporation into bacteria extracted using homogenisation-centrifugation. Bulk soil bacterial activity was low compared with that of rhizosphere bacteria. Nitrogen addition did not affect the activity of the bulk soil bacteria, indicating that the activity was not nitrogen limited. The thymidine and leucine incorporation rates of rhizosphere bacteria decreased when ammonium or ammonium + nitrate was applied compared with the non-amended controls. No effect on bacterial activity was found following nitrate addition. There was a significant positive correlation between rhizosphere bacterial activity and rhizosphere pH. Shoot length following ammonium treatment was significant lower than in the non-amended control, while nitrate and ammonium + nitrate addition had no effect. This indicates that the varying effects due to nitrogen sources on rhizosphere bacterial activity were not due to effects on plant growth.