We explore the transformation of antenna to leg in Drosophila melanogaster, using ectopically expressed transgenes with heat shock promoters: heat shock Antennapedia, heat shock Ultrabithorax, and heat shock mouse Hox A5. We determined the frequency of transformation of several leg markers in response to Antennapedia protein delivered by heat shock at different times and doses. We also studied stage-specific responses to the transgene, heat shock mouse Hox A5. Results show that each marker has its own stage and dose-specific pattern of response. The same marker could pass through a period of high-dose inhibition followed by a dose-independent response and then a positive dose-dependent phase. The heat shock-induced transgenes and spineless aristapedia transformed the apterous enhancer trap antenna disc expression pattern toward the pattern found in leg discs. These results are considered in relation to developmental competence-the ability of developing tissue to respond to internal or external influences. The results suggest that all genes tested interact with the same competence system and that at least two classes of mechanisms are associated with antenna to leg transformation: one comprises global mechanisms that permit transformation over approximately 24 hr; the second class of mechanisms act very locally and are responsible for changes in dose response on the order of 4-8 hr.