Gravitropism is an important factor in determining root architecture and directs roots to grow downward in soil and thus may have a great affect on the uptake of water and mineral ions required for plant growth and development. Ammonium profoundly affects root architecture of Arabidopsis thaliana. Despite the inhibitory effects of high ammonium on root length and the number of lateral roots, however, very little is known concerning the influence of ammonium on root growth direction. Experiments were performed with Arabidopsis thaliana using indoor agar media for a simulation to study the effects of high ammonium on root growth direction. It was found that NH4+ stress not only inhibited plant growth, but also affected root growth direction. Kinetics results for a 30-h assay window showed that 10 mmol/L (NH4)2SO4 delayed root gravitropism, while 30 mmol/L (NH4)2SO4 significantly reduced root gravitropic curvature. Reduction the nutrients in basic medium, root agravitropic response induced by NH4+ occurred at a lower concentration. Adding Ca(NO3)2 partially restored the inhibition of NH4+ on root elongation, but had no effects on NH4+ induced root gravitropic angle. When NH4+ was supplied separately to different parts of the plants, the effect of NH4+ on root gravitropism was mainly from the root-supplied NH4+. These results preliminarily indicate that the influence of ammonium on root growth direction is partially independent of its effect on root elongation and is not due to the de?ciency of cations such as Ca2+, but may be related with the alteration of root physiological processes when root exposed to high NH4+.