The duration of ERK1/2 activation influences the nature of the biological response to agonist. Members of the AP-1 transcription factor family are well known targets of the ERK1/2 pathway and are expressed in a temporally coordinated fashion during cell cycle re-entry. In CCl39 fibroblasts, sustained ERK1/2 activation is required for the expression of Fra-1, Fra-2, c-Jun and JunB, whereas expression of c-Fos is still strongly induced even in response to transient ERK activation. However, the significance of this pattern of expression for AP-1 activity has not been addressed. Here we show that growth factor stimulated activation of the C-terminal c-Fos transactivation domain (TAD) serves as a sensor for ERK1/2 signal duration whereas the c-JunTAD is not responsive to growth factors. In addition, sustained ERK1/2 activation determines the duration of increases in AP-1 DNA binding complexes as well as their qualitative make up. Finally, this is reflected in both the duration and quantitative transcriptional output of stably integrated AP-1 reporter constructs, indicating that AP-1 activity is finely tuned to ERK1/2 signal duration. These results provide new insights into the importance of ERK1/2 signal duration in the regulation of AP-1 and provide an explanation for how differences in signal duration can lead to both quantitative and qualitative changes in gene expression.