Hydra regenerate throughout their life. We previously described early modulations in cAMP-response element-binding protein(CREB) DNA-binding activity during regeneration. We now show that the Ser-67 residue located in the P-box is a target for post- translational regulation. The antihydra CREB antiserum detected CREB-positive nuclei distributed in endoderm and ectoderm, whereas the phosphoSerl33-CREB antibody detected phospho-CREB-positive nuclei exclusively in endodermal cells. During early regeneration, we observed a dramatic increase in the number of phospho-CREB-positive nuclei in head-regenerating tips, exceeding 80% of the endodermal cells. We identified among CREB-binding kinases the p80 kinase, which showed an nhanced activity and a hyperphosphorylated status during head but not foot regeneration. According to biochemical and immunological evidence, this p80 kinase belongs to the Ribosomal rotein S6 kinase family. Exposure to the U0126 mitogen-activated protein kinase kinase inhibitor inhibited head but not foot regeneration, abolished CREB phosphorylation and activation of the early gene HyBra 1 in head-regenerating tips. These data support a role for the mitogen-activated protein kinase/ribosomal protein S6 kinase/CREB pathway in hydra head organizer activity.