Abstract: Solution culture experiments were carried out to study the effects of different medium pH(pH 4.5,pH 6.5) and N forms(NH4+-N,NO3--N) on arenchyma formation in roots of five rice genotypes.The five genotypes were as follows: conventional indica(Yangdao No.6),hybrid indica(Shanyou 63),conventional japonica(Nongken 57),hybrid japonica(Siyou 917) and upland rice(Baxi upland rice). Results showed that the effects of N forms(NH4+-N,(NO3--N)) and medium pH(pH 4.5,pH 6.5) on arenchyma formation in roots varied with rice genotypes.Low pH(4.5) had negligible effects on aerenchyma formation in roots of japonica(conventional japonica,hybrid japonica) when the(NH4+-N) was the sole N source;while low pH level(pH 4.5) stimulated indica rice(conventional indica,hybrid indica) to form aerenchyma in roots in case providing the same N source.Under the condition of NO3--N being the sole N source,near neutral pH(pH 6.5) was more favorable to form aerenchyma in the root of japonica rice and upland rice,while the effect of medium pH on the aerenchyma formation of indica rice did not show a statistical significance.Under the low pH condition with NH4+-N as the sole N source, aerenchyma could be formed much easier in the root cortex of rice.This might be an adaptable mechanism of plant to decrease toxicity of ammonium accumulation under low pH condition.