We examined the effect on etiolated Arabidopsis thaliana seedlings of a wide variety of factors that are known to induce ethylene in other plant tissues. Auxin, cytokinin, brassinosteroid and cupric ion were found to highly elevate ethylene production in these seedlings, but several other signaling compounds, as well as wounding and mechanical stimulation, had little or no effect. A mutant that disrupts the ACS5 gene (cin5) was partially defective in the induction of ethylene in the presence of brassinosteroids, suggesting a role for this isoform in mediating this response. Cytokinin displayed a synergistic interaction with both brassinosteroid and auxin, while the other interactions tested were essentially additive. Auxin and cytokinin have been shown to act synergistically to elevate ethylene biosynthesis in many other plant tissues. We show that the synergism between cytokinin and auxin in Arabidopsis is due to an enhancement of the effects of auxin, but not by increased elevation of ACS4 mRNA levels. These results suggest that cytokinin acts post-transcriptionally to increase ACS4 function, which, coupled with the observation that auxin elevates ACS4 mRNA levels, accounts for the synergistic interaction.