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The effect of ethylene exposure time on leaf senescence was studied in Arabidopsis accessions Ler-0, Col-0 and Ws-0 and in several old (onset of leaf death) mutants representing different genetic loci. Leaf senescence was observed in 24-day-old plants exposed to ethylene for 3 to 16 days. For ethylene treatments up to 12 days, a longer ethylene exposure time resulted in more senescing leaves in wild type plants. However, a 16-day ethylene treatment reduced senescence symptoms. old mutants differentially responded to varied ethylene exposure times. In old3, old9, old12 and old14 mutants, the same number of leaves turned yellow, regardless of the ethylene exposure time. In old5 and old13, increasing ethylene exposure time from 6 to 16 days resulted in the same number of yellow leaves, but less leaves yellowed in a 3-day treatment. The amounts of senescing leaves in old1-1 and old11 mutants decreased with increases in ethylene exposure time from 6 to 16 days. Thus, multiple OLD genes may act to restrict the induction of senescence by ethylene in an ethylene exposure time-dependent manner. The data substantiate the notion that there is a complex interaction between leaf age and ethylene involving multiple genetic loci.