The endogenous glycogen synthase in extracts from mouse skeletal muscle, liver and brain bound specifically to SAPK2b (stress-activated protein kinase 2b)/p38b, but not to other members of the group of SAPK/p38 kinases. Glycogen synthase was phosphorylated in vitro more efficiently by SAPK2b/p38b than by SAPK2a/p38a, SAPK3/p38g or SAPK4/p38d. SAPK2b/p38b phosphorylated glycogen synthase in vitro at residues Ser644, Ser652, Thr718 and Ser724, two of which (Ser644 and Ser652) are also phosphorylated by glycogen synthase kinase 3. Thr718 and Ser724 are novel sites not known to be phosphorylated by other protein kinases. Glycogen synthase becomes phosphorylated at Ser644 in response to osmotic shock; this phosphorylation is prevented by pretreatment of the cells with SB 203580, which inhibits SAPK2a/p38a and SAPK2b/p38b activity. In vitro, phosphorylation of glycogen synthase by SAPK2b/p38b alone had no significant effect on its activity, indicating that phosphorylation at residue Ser644 itself is insufficient to decrease glycogen synthase activity. However, after phosphorylation by SAPK2b/p38b, subsequent phosphorylation at Ser640 by glycogen synthase kinase 3 decreased the activity of glycogen synthase. This decrease was not observed when SAPK2b/p38b activity was blocked with SB 203580. These results suggest that SAPK2b/p38b may be a priming kinase that allows glycogen synthase kinase 3 to phosphorylate Ser640 and thereby inhibit glycogen synthase activity.