In rice (Oryza sativa L.) OsAMT1.1 is the most active and / or most N-responsive gene responsible for high-affinity NH₄⁺ transport (HATS) activity. We measured ¹³NH₄⁺ influx and plant biomass in transgenic overexpression lines and two wild type cultivars of rice, Jarrah and Taipei, with one or more copies of OsAMT1.1. ¹³NH₄⁺ influx was higher for the overexpression lines of Jarrah line when grown at 10 µm external NH₄⁺ concentration, but not for the overexpression lines of Taipei. For seedlings grown at 2 mm external NH₄⁺ concentration Jarrah lines 77-1 and 75-4 showed an increased influx; however, two overexpression lines of Taipei showed reduced influx rates. The biomasses of the transgenic lines grown at low and high external NH₄⁺ concentrations were either reduced or showed no statistically significant differences compared with wild type lines. While ¹³NH₄⁺ influx into roots of Jarrah line 75-4 grown at 10 µm external NH₄⁺ concentration was significantly higher than in wild type, measurements of ¹³NH efflux revealed no differences, and thus net uptake of NH₄⁺ was higher in this overexpression line.