We investigated the differential expression of seven ACC synthase and two ACC oxidase genes in mung bean leaves. Among these, only ACS2, ACO1, and ACO2 were expressed in etiolated leaves. When seedlings were de-etiolated, the expression level of ACOI decreased by 65%, expression of ACOS disappeared, while that of AnS was unchanged. For de-etiolated leaves treated with NaCl in the dark, the photochemical efficiency of PSll was not altered, and no genes were newly induced within the first 12 h. However in the presence of light, transcripts of ACS1, ACS3, ACS4, and ACS6 were newly accumulated, and the expression levels of ACS2 and ACO1 were increased. The kinetics of transcript accumulation in response to methyl viologen (MV) treatment in the light were similar to those observed in the NaCl-treated leaves in the presence of light, suggesting that changes in the Hatter were caused by oxidative, rather than saline, stress. However transcripts from most of the genes began accumulating more slowly in MV·treated leaves, with those of ACS1 and ACS6 accumulating in much lower amounts than in NaCl-treated leaves. The reverse was true for ACS4 transcripts.