Transcription of the c-fos gene is transiently activated to generate large amounts of unstable c-fos RNA when quiescent fibroblasts are stimulated by polypeptide mitogens or whole serum. A cloned human c-fos gene (c-fosH) transfected into mouse fibroblasts is regulated in a similar manner. An element essential for transcription activation is located between nucleotides -332 and -276 relative to the mRNA cap site. This element has properties similar to those of previously characterized transcription enhancer elements. However, replacement of the 5' activating element by enhancers from SV40 or Moloney murine leukemia virus does not allow regulated c-fosH expression. The study of fusion genes showed that in addition to the 5' activating element, transient accumulation of c-fosH RNA following serum stimulation requires sequences at the 3' end of the c-fosH gene.