In hepatocytes incubated in the presence of 20 mM glucose, strong stimulation of glycogen synthesis by LiCl was observed in cells from both normal and streptozotocin-induced diabetic rats. This stimulation was concentration- and time-dependent, and it was related to an increase in the glycogen synthase activity ratio. No effects of LiCl on glycogen phosphorylase activity were detected under these conditions. When hepatocytes were incubated with LiCl and insulin together, an additive effect on glycogen synthesis from glucose was observed. However, when hepatocytes were incubated in the presence of gluconeogenic precursors, such as lactate and pyruvate, and then with LiCl, no effect on glycogen accumulation was observed. Under these conditions, simultaneous activation of glycogen synthase and glycogen phosphorylase occurred. These results indicate that lithium ions, like insulin, are able to stimulate glycogen synthesis from glucose in diabetic rat hepatocytes, although the mechanisms of action of the two effectors on hepatic glycogen metabolism are probably different.