In this study, we examined the effect of phosphorylation on GLUT-4 function in isolated rat adipocytes. Adipocytes labeled with 32P for 2 h were incubated with parathyroid hormone (PTH) (20 ng/ml) for 60 min and then exposed to insulin (25 ng/ml) for an additional 30 min. 32P-GLUT-4 was immunoprecipitated from the plasma membrane and low density microsomal fractions, and its degree of phosphorylation was determined by autoradiography and densitometry. Results were expressed as 32P-GLUT-4 specific activity (phosphorylation/unit of protein). GLUT-4 intrinsic activity was measured using [14C]2-deoxyglucose uptake in plasma membrane vesicles. PTH significantly increased GLUT-4 phosphorylation and eliminated the insulin-stimulated dephosphorylation of GLUT-4. Western blotting revealed normal distribution of GLUT-4 before and after insulin stimulation in control and PTH-treated cells, suggesting that phosphorylation of GLUT-4 does not interfere with its recruitment to the plasma membrane. In contrast, intrinsic activity of phosphorylated GLUT-4 was significantly reduced (p < 0.01). Preincubation of adipocytes with calcium channel blocker (nitrendipine) and cyclic AMP antagonist (RpcAMP) restored GLUT-4 intrinsic activity in the PTH-treated cells. In several experiments, GLUT-4 was phosphorylated in vitro in plasma membrane vesicles isolated from normal adipocytes exposed to insulin. This in vitro phosphorylation reduced GLUT-4 intrinsic activity by approximately 35% (p < 0.01). We conclude that phosphorylation of GLUT-4 significantly impairs the ability of insulin to stimulate its intrinsic activity.