Aux͞IAA gene family members were first identified by their rapid transcriptional increase in response to auxin. Auxin͞indole-3-acetic acid protein (Aux͞IAA) luciferase (LUC) fusions expressed in Arabidopsis under control of a non-auxin-responsive promoter were used to monitor the effect of auxin on protein abundance independent of transcriptional regulation by auxin. After 2 hr in the presence of 1 M exogenous dichlorophenoxyacetic acid (2,4D), a synthetic auxin, the levels of pea IAA6 (PSIAA6) and Arabidopsis IAA1 LUC activity were 35% and 67%, respectively, of mocktreated genetically identical seedlings, whereas the activity of LUC alone from equivalently treated seedlings remained unaltered. The steady-state level of an Aux͞IAA fusion protein lacking domain II, one of the conserved domains found in all Aux͞IAA proteins, was not reduced in the presence of auxin. Higher levels of exogenous auxin were required to affect the steady-state level of the PSIAA6::LUC fusion with a point mutation in domain II. A 13-aa consensus sequence from domain II fused to LUC created an auxin-responsive fusion protein. The change in steady-state levels in response to auxin is extremely rapid, with a decrease in LUC activity detectable by 2 min after auxin application. Direct half-life measurements show that the decrease caused by exogenous auxin is due to the decrease in fusion protein half-life. These results suggest that auxin rapidly modulates the degradation rate of Aux͞IAA proteins, with higher levels of auxin increasing the proteolytic rate of Aux͞IAA family members.