Rsk is a 90-kDa protein kinase that is activated by phosphorylation by MAP kinase at the end of a well-established signaling cascade. Rsk has two conserved catalytic kinase domains. The amino terminal kinase domain is responsible for phosphorylation of exogenous substrates. The carboxyl terminal domain of rsk has a basal autophosphorylation activity which can be detected when recombinant protein is incubated with [gamma-32P]ATP. The manner in which rsk activity is controlled by site specific phosphorylation is largely unknown. We show that rsk can autophosphorylate through an intermolecular mechanism. Autophosphorylation occurs primarily on serine 380, in a highly conserved region of rsk between its two kinase domains. That site of autophosphorylation is similar to sites found in other serine/threonine kinases, which are also regulated by phosphorylation at that corresponding site. The carboxyl terminal kinase domain of rsk becomes a potential candidate kinase involved in phosphorylating and regulating the activity of those other kinases through their conserved domains.