In order to investigate the molecular basis of high-affinity ammonium absorption by roots of rice plants (Oryza sativa subspecies indica) the expression patterns of three members of the AMT1 family of genes in rice seedling roots in response to altered nitrogen provision and diurnal changes in irradiance were examined. The 13NH4+ influx and transcript levels of OsAMT1.1 in roots decreased several fold within 48 h when plants acclimated to 10 micro m external NH4+ for 3 weeks were transferred to 10 mm NH4+. Likewise when plants acclimated in 10 mm NH4+ were transferred to 10 micro m NH4+, there was an equally rapid up-regulation of OsAMT1.1 and 13NH4+ influx in the roots. Changes in transcript abundance of OsAMT1.2 following these treatments were approximately 50% less than in OsAMT1.1, and changes of OsAMT1.3 expression were even less. By contrast, in response to the diurnal changes of irradiance, root transcript abundance of OsAMT1.3 and 15NH4+ influx increased approximately three-fold late in the photoperiod, whereas OsAMT1.1 and OsAMT1.2 exhibited only modest changes. The present results suggest that high-affinity NH4+ influx is differentially regulated at the transcriptional level through the expression of three members of the OsAMT1 family of genes in roots of rice seedlings in response to changes of N status and daily irradiance. In general, these findings are in agreement with earlier observations in Arabidopsis and tomato.