Methodology has been developed to study the redox state of thiol proteins in cells. Initial results of the application of these methods indicate that growth-factor induced rise in cytosolic calcium causes the inhibition of thioredoxin reductase and the oxidation some 60 % of the cellular thioredoxin. These are the first results that show that early in signal transduction there is a transient state of oxidative pressure which may cause other vicinal thiol proteins to become oxidized to the disulfides. Many phosphotyrosine phosphatases have vicinal thiols and therefore, we are determining whether these key regulators are transiently inhibited by the calcium-induced oxidation. Other cell cycle dependent regulatory steps are being studied from the point of view of redox regulation.