Treatment of cells with okadaic acid, a protein phosphatase inhibitor, leads to an insulin-resistant state without modification in the tyrosine kinase activity of the receptor toward exogenous substrates. In 3T3-L1 adipocytes, okadaic acid induced a similar dose-dependent inhibition of the insulin effect on deoxyglucose uptake, phosphatidylinositol 3-kinase (PI 3-kinase) activation, and insulin receptor substrate (IRS) 1 tyrosine phosphorylation. Simultaneously, in okadaic acid-treated 3T3-L1 adipocytes, the reduced IRS 1 tyrosine phosphorylation was linked to a decrease in its electrophoretic mobility due to phosphorylation on serine/threonine residues. This phosphorylation appeared to result from the activation of cytosolic kinase(s). Furthermore, using in vitro reconstitution, we show that, compared to IRS 1 immunopurified from untreated cells, the IRS 1 obtained from okadaic acid-treated cells had a reduced capacity to be phosphorylated by insulin receptors and, concomitantly, to bind PI 3-kinase. Taken together these data suggest that serine/threonine phosphorylation of IRS 1 induced by okadaic acid reduces the ability of the insulin receptor to phosphorylate IRS 1 and to dock one of its interacting molecules, i.e. PI 3-kinase. Finally, the inhibitory effect of okadaic acid on the stimulatory action of insulin on glucose transport suggests that the serine/threonine phosphorylation of IRS 1 might represent a key regulatory mechanism of insulin action.