Background: The mitogen-activated protein (MAP) kinases contribute to altered cell growth and function in a variety of disease states. However, their role in the endothelial complications of diabetes mellitus remains unclear. Methods: Human endothelial cells were exposed for 72 hours to 5 mM (control) or 25 mM (high) glucose or 5 mM glucose plus 20 mM mannitol (osmotic control). The roles of p38 and p42/44 MAP kinase in the high glucose induced growth effects were determined by assessment of phosphorylated MAP kinase and their downstream activators by Western blot, and by pharmacological inhibition of these MAP kinases. Results were expressed as percentage ± SEM of control. Results: High glucose increased the activity of total and phosphorylated p38 MAP kinase (P < 0.001) and p42/44 MAP kinase (P < 0.001). Co-exposure of p38 MAP kinase blocker with high glucose reversed the antiproliferative but not hypertrophic effects associated with high glucose conditions. TGF1 increased the levels of phosphorylated p38 MAP kinase and p38 MAP kinase blockade reversed the antiproliferative effects of this cytokine. The high glucose induced increase in phosphorylated p38MAP kinase was reversed in the presence of TGF1 neutralising antibody. Although hyperosmolarity also induced antiproliferation (P< 0.0001) and cell hypertrophy (P< 0.05), there was no change in p38 activity, and therefore inhibition of p38MAP kinase had no influence on these growth responses. Blockade of p42/44 MAP kinase had no effect on the changes in endothelial cell growth induced by either high glucose or hyperosmolarity. Conclusion: High glucose increased p42/44 and p38 MAP kinase activity in human endothelial cells, but only p38 MAP kinase mediated the antiproliferative growth response through the effects of autocrine TGF1. High glucose induced endothelial cell hypertrophy was independent of activation of the MAP kinases studied. In addition, these effects were independent of any increase in osmolarity associated with high glucose exposure.