Diadenosine tetraphosphate (AP4A) is a diadenosine polyphosphate. Diadenosine polyphosphates (APnAs, n=3-6) are a family of endogenous vasoactive purine dinucleotides which have been isolated from thrombocytes. APnAs have been demonstrated to be involved in the control of vascular tone as well as the growth of vascular smooth muscle cells and hence, possibly, in atherogenesis. APnAs isolated substances are Ap3A, Ap4A, Ap5A, and Ap6A. APnAs are naturally occurring substances that facilitate tear secretion; they are released from the corneal epithelium, they stimulate tear production and therefore they may be considered as physiological modulators of tear secretion. The APnAs were discovered in the mid-sixties in the course of studies on aminoacyl-tRNA synthetases (aaRS). APnAs have emerged as intracellular and extracellular signalling molecules implicated in the maintenance and regulation of vital cellular functions and become considered as second messengers. Great variety of physiological and pathological effects in mammalian cells was found to be associated with alterations of APnAs. APnAs are polyphosphated nucleotidic substances which are found in the CNS and are known to be released in a calcium-dependent manner from storage vesicles in brain synaptosomes. AP5A is a specific adenylate kinase inhibitor in the hippocampus, decreasing the rate of decomposition of ADP and the formation of ATP; a pathway that influences the availability of purines in the central nervous system. AP4A is the only APnA that can induce a considerable increase in [Ca2+] in endothelial cells, indicating that its vasoactive effects are comparable to the known effects of arginine vasopressin, Angiotensin II, and ATP. AP4A is a ubiquitous ApnA is a signal molecule for DNA replication in mammalian cells. AP4A is a primer for oligoadenylate synthesis catalyzed by interferon-inducible 2-5A synthetase. AP4A is an avid inhibitor of eosinophil-derived neurotoxin (EDN). EDN is a catalytically proficient member of the pancreatic ribonuclease superfamily secreted along with other eosinophil granule proteins during innate host defense responses and various eosinophil-related inflammatory and allergic diseases. The ribonucleolytic activity of EDN is central to its antiviral and neurotoxic activities and possibly to other facets of its biological activity.