c-Jun expression is down-regulated in v-Jun- and c-Jun-transformed chicken embryo fibroblasts. The down-regulation is a specific consequence of high Jun expression suggesting that c-jun, like several other immediate early genes, is subject to negative autoregulation. The work in this thesis has attempted to define the consequences of and the molecular basis for Jun-mediated autorepression in vivo. c-Jun is the sole or predominant Jun family protein expressed in primary CEFs. Repression of endogenous p39 c-Jun in ASV17-transformed cultures results in the replacement of c-Jun-containing AP-1/TRE binding complexes with v-Jun-containing alternatives. The absence of auxiliary Jun family proteins facilitates the displacement and may contribute to the unique transforming activites of v-Jun (and c-Jun) in avian cells. v-Jun-mediated auto-repression is primarily directed at the level of transcription and correlates with specific changes in occupancy at the proximal junTRE and adjacent junRSRE binding sites in the c-jun promoter. In normal asynchronous cultures specific binding factors compete for the adjacent junTRE and junRSRE regulatory elements. In ASV17-transformed cells the junTRE is exclusively occupied by v-Jun-containing complexes and endogenous c-jun expression is down-regulated. The absence of junRSRE occupancy in ASV17-transformed cells is associated with high levels of the v-Jun oncoprotein which physically disrupt or inhibit the binding activity of junRSRE-specific complexes. Mutually exclusive binding at the junTRE and junRSRE or a Jun-dependent sequestration of specific accessory factors have been proposed to direct the single pattern of occupancy in ASV17-transformed cells and to, thereby, contribute to the down-regulation of endogenous c-jun expression.