The vascular endothelium is the source of a number of vasodilator and vasoconstrictor autacoids and is thus a key regulator of vascular homeostasis. We studied the effects of altering the balance between protein tyrosine kinase and phosphatase activity on Ca<sup>2+</sup> signalling and phosphotyrosine levels in cultured human endothelial cells, as well as on autacoid production in native endothelial cells. In isolated segments of rabbit aorta and carotid artery, as well as in bovine coronary arteries, the tyrosine phosphatase inhibitors phenylarsine oxide (PAO) and sodium orthovanadate initiated endothelium-dependent relaxations which could be attributed to the release of nitric oxide and the endothelium-derived hyperpolarizing factor. In cultured endothelial cells incubation with PAO resulted in a time-dependent accumulation in 6-keto prostaglandin F<sub>1α</sub>, the stable metabolite of prostacyclin, as well as in an increase in the intracellular concentration of free Ca<sup>2+</sup> ([Ca<sup>2+</sup>]i). Inhibition of tyrosine kinases attenuated both the PAO-induced relaxation and the increase in endothelial [Ca<sup>2+</sup>]i. Western blot analysis of endothelial cells treated with the tyrosine phosphatase inhibitors revealed a time-dependent increase in the tyrosine phosphorylation of a series of bands in both the Triton X-100-soluble and Triton X-100-insoluble (cytoskeletal) fractions. These observations suggest that alterations in cellular levels of phosphotyrosine may have profound effects on vascular homeostasis by modulating Ca<sup>2+</sup> signalling and autacoid production in endothelial cells.