Activation of rapidly reversible temperature-sensitive (ts) v-Src in quiescent chicken embryo fibroblasts (CEFs) results in both morphological transformation and exit from G0 to G1, resulting in mitosis. This phenomenon permits examination of cellular responses very soon after activating the oncoprotein, and we have used this to study changes in endogenous AP-1, and the regulation of its major components, in the first few hours after activating v-Src. This approach contrasts with a number of studies that have demonstrated enhanced activity of exogenously added AP-1 components in cells transformed by v-Src. Reactivation of a membrane-associated tyrosine kinase (tsRCAN-29) results in a several-fold increase in AP-1 DNA binding and a similar increase in the activity of an AP-1-responsive reporter soon after temperature shift. c-Jun and c-Fos are regulated at a number of levels in response to both stimuli. In quiescent RCAN-29-infected CEFs stimulated into cycle by shift to permissive temperature, c-fos transcripts are elevated by 15 min and remain above basal level for at least 4 h. Serum induces much greater elevation of c-fos transcripts, although this response is transient. Despite the difference in magnitude of the transcript responses, the stimulation of nuclear c-Fos protein is similar in both serum and v-Src-stimulated cultures. No elevation in c-jun transcripts or nuclear c-Jun protein level is evident in v-Src-stimulated quiescent CEFs. However, there is an early change in the tryptic phosphopeptide map of p39 c-Jun in response to both v-Src and serum. Upon stimulation we observed a novel redistribution of phosphate in the carboxy-terminal tryptic phosphopeptide that may be responsible in part for the increase in AP-1 DNA binding. Phosphorylation of amino-terminal serines 63 and 73 on peptides Y and X, believed to be responsible for regulation of the transactivation function of c-Jun, is constitutively high in resting CEF cultures; stimulation with serum or v-Src results in only a modest increase in phosphorylation at these sites. Significantly, reactivation of a non-myristylated, transformation-defective version of the tsRCAN-29 v-Src protein (RCAN-29A2) is unable to induce resting CEFs to re-enter cycle. In addition, this mutant fails to induce early increases in AP-1 activity, implying that these nuclear changes require crucial signalling events at the cell periphery, and that these events correlate with the biological consequences of expression of v-Src.