Rap1 GTPase activation by its cAMP responsive nucleotide exchange factor Epac present in endothelial cells increases endothelial cell barrier function with an associated increase in cortical actin. Here, Epac1 was shown to be responsible for these actin changes and to colocalize with microtubules in human umbilical vein endothelial cells. Importantly, Epac activation with a cAMP analogue, 8-pCPT-2'O-Me-cAMP (O-Me-cAMP) resulted in a net increase in the length of microtubules. This did not require cell-cell interactions or Rap GTPase activation and was attributed to microtubule growth as assessed by time-lapse microscopy of HUVEC expressing fluorophore-linked microtubule plus-end marker end-binding protein 3. An intact microtubule network was required for Epac mediated changes in cortical actin and barrier enhancement but was not required for Rap activation. Finally, Epac activation reversed microtubule dependent increases in vascular permeability induced by TNF-alpha and TGFbeta. Thus Epac can directly promote microtubule growth in endothelial cells. This, together with Rap activation leads to an increase in cortical actin, which has functional significance for vascular permeability.