To evaluate possible mechanisms by which insulin inhibits hepatic apolipoprotein B (apoB) secretion, we incubated primary cultures of rat hepatocytes with sodium orthovanadate, a phosphotyrosine phosphatase inhibitor and insulin-mimetic agent. Vanadate (10 microM) and insulin (10 nM) inhibited the medium accumulation of apoB (secretion) by 21 and 37%, respectively, without increasing intracellular apoB. The effects of insulin and vanadate together were not additive. Both insulin and vanadate enhanced intracellular glycogen accumulation by 82 and 37%, respectively. Unlike insulin, vanadate, at a concentration that inhibited apoB secretion (10 microM), had no effect on intracellular lipogenesis, inhibited the secretion of newly synthesized hepatic proteins, and had a delayed onset and termination of action on inhibition of apoB secretion. At higher concentrations (40 and 80 microM), vanadate stimulated intracellular lipogenesis. In conclusion, our data indicate that vanadate mimics insulin action in hepatocytes with regard to the inhibition of medium accumulation of apoB. These data are consistent with the hypothesis that inhibition of apoB secretion may be secondary to an increase in phosphotyrosine content at its site of synthesis. The kinases responsible for this effect have not been identified. Several effects of vanadate, however, are different from those of insulin, suggesting a differential sensitivity to vanadate, a divergence of the signal transfer by insulin and vanadate at the insulin-receptor or postreceptor level, or both.