Although a large number of protein kinases/phosphatases involved in insulin's actions have been characterized recently, relatively few of the downstream phosphoproteins have been identified. We have employed two-dimensional gel electrophoresis-based proteome analysis to investigate the insulin-evoked phosphorylation cascade in rat soleus muscle. Insulin reproducibly increased phosphorylation of a 20-kDa protein with a pI value of 6.0, which was identified subsequently as a phospho-isoform of P20, a small heat-shock-related protein. The adenylyl cyclase activator, forskolin, decreased phosphorylation of this P20 isoform and increased phosphorylation of another two P20 isoforms, with pI values of 5.9 and 5.6. Two-dimensional peptide mapping revealed that the phospho-peptides of these three P20 isoforms are different. In contrast to its action in soleus muscle, insulin decreased phosphorylation of the P20 isoform with pI 6.0 and increased phosphorylation of the two isoforms with pI values of 5.9 and 5.6 in vascular smooth muscle. This effect is similar to that induced by vasodilatory stimuli, suggesting that insulin could exert its vasodilatory action by affecting phosphorylation of P20. In summary, these results demonstrate that insulin differently modulates phosphorylation of P20 in skeletal and smooth muscle, and suggest that P20 could be a potential modulator of insulin's functions in these tissues.