Nicotine and nornicotine are the two principal alkaloids in cultivated tobacco ( Nicotiana tabacum L.) with more than 95% of the total alkaloid often being nicotine and 3.5% or less being nornicotine. In some plants a substantial amount of nicotine may be demethylated to nornicotine. The nicotine to nornicotine conversion is undesirable since high levels of nornicotine content adversely affects leaf quality. A series of experiments were initiated to evaluate the effect of ethylene treatment on nicotine to nornicotine conversion in greenhouse and field grown burley tobacco for the purpose of identifying converter plants prior to curing. In greenhouse grown converter plants of lines 78379 and TN90, the percent nicotine conversion in all ethylene treated leaves 11days after treatment was greater than 94%, while only 66% and 42% of the nicotine was converted to nornicotine in normally air-cured leaves of 78379 and TN 90, respectively. The stimulating effect of ethylene treatment on nicotine conversion was similar in leaves from different stalk position with different physiological age. Also, ethylene caused a similar response on nicotine conversion in detached leaves from young seedlings. Removal of ethylene by using vacuum or an ethylene absorbent resulted in significant inhibition to the conversion of nicotine to nornicotine. Conversion levels comparable to non-converters were observed after converter leaves were cured in vacuo for 21 days. Percent conversion was 5.6% after curing with ethylene absorbent for 23 days compared to 45% conversion in leaves cured without ethylene absorbent. Ethylene treatment was effective in identifying converter plant at early plant growth stage in the field. There was no measurable effect of ethylene treatment on nornicotine demethylation in non-converter plants.