In fibroblast cells, cAMP antagonizes growth factor activation of ERKs and cell growth via PKA and the small G protein Rap1. We demonstrate here that PKA's activation of Rap1 was mediated by the Rap1 guanine nucleotide exchange factor C3G, the adaptor Crk-L, the scaffold protein Cbl, and the tyrosine kinase Src. Src was required for cAMP activation of Rap1 and the inhibition of ERKs and cell growth. PKA activated Src both in vitro and in vivo by phosphorylating Src on serine 17 within its amino terminus. This phosphorylation was required for cAMP's activation of Src and Rap1, as well as cAMP's inhibition of ERKs and cell proliferation. This study identifies an antiproliferative role for Src in the physiological regulation of cell growth by cAMP.