The molecular chaperone Hsp90 affects the function and fate of a number of signaling molecules. We have investigated the Hsp90 requirement for constitutive and inducible activity of the IkappaB kinase (IKK) complex and of NF-kappaB. Inhibition by the Hsp90 ATPase inhibitors, geldanamycin (GA) and radicicol (RC), revealed that Hsp90 controls IKKs at two levels, inducibility of enzymatic activity and biogenesis, which can be discriminated by short- and long-time GA incubation, respectively. Short-time inhibition of Hsp90 resulted in impaired IKK kinase activation by TNFalpha, IL-1beta or phorbolester PMA. Furthermore, GA inhibited constitutive activation of IKK and NF-kappaB in Hodgkin's lymphoma cells. Hsp90 function was also required for trans- and autophosphorylation of transfected IKKbeta. GA exposure for several hours resulted in a downmodulation of IKK complex alpha, beta and gamma subunits to various extent. Proteasome inhibition interfered with GA mediated IKK depletion and Hsp90 inhibition induced polyubiquitination of IKKalpha and beta during protein synthesis. In fact, GA blocked biogenesis of IKKalpha and IKKbeta but did not interfere with post-translational turnover. Together, these results define a dual requirement for Hsp90 as a regulator of NF-kappaB signaling by its general involvement in IKK activation and by its role in IKK homeostasis.