Ethionine has been shown by Schrank (12) to be an inhibitor of auxin-induced elongation in the Avena coleoptile. Since the inhibition can be reversed by methionine, it has been concluded that ethionine acts by affecting some facet of methionine metabolism. Work with animal tissues has shown that there are two ways in which this can occur: ethionine can antagonize the incorporation of methionine into proteins and thus block protein synthesis (14) or it can inhibit transmethylation (13). Recent work has suggested that ethionine may be inhibiting elongation in the Avena coleoptile by interfering with transmethylation. Auxin has been shown to cause an increase in the rate of transfer of methyl groups from methionine to cell wall pectin in this tissue (11). If this transmethylation is obligatory for auxin-action, inhibition of this methylation by ethionine would lead to an inhibition of elongation. This theory is onlv tenable if ethionine actually does inhibit cell wall methylation and this has not been previously demonstrated. It was the purpose of this investigation to see if such an inhibition could be found. Schrank has shown that ethionine inhibits total elongation (12). Total elongation (TE) is the sum of reversible (ES) and irreversible elongation (IE). The inhibition of TE by ethionine has now been separated into inhibitions of ES and IE and the rapidity of each inhibition has been determined from timecourse studies. In this paper, the term "cell wall methylation" is used to describe the transfer of methyl groups from methionine to the hot-water-soluble fraction of the cell wall. The effect of ethionine on the methylation of cold-water-soluble pectin has not been examined in this study.