AbstractRecent evidence suggests that intramolecular autophosphorylation is responsible for the tyrosine phosphorylation (pY) of residues 279 or 216 of glycogen synthase kinase–3 (GSK‐3α or β), an event that appears to play an important role in regulating this kinase. This provocative hypothesis was based on the capacity of certain nonselective GSK‐3 inhibitors to alter both the activity of GSK‐3 and its pY. Inhibitors of GSK‐3 are not always capable of preventing this tyrosine phosphorylation, which may require an extended period of time. For example, although lithium chloride inhibits GSK‐3 activity, this inhibition does not alter its pY content. Furthermore, even when GSK‐3 activity is impaired, GSK‐3 pY can still be modified by physiological or pharmacological agents. Taken together, these data indicate that GSK‐3 kinase activity is not necessarily correlated with the extent of GSK‐3 pY. We hypothesized that some as‐yet‐unidentified tyrosine kinases and phosphatases may also regulate this kinase. © 2007 Wiley‐Liss, Inc.