['To examine diadenosine tetraphosphate (Ap(4)A) for its ability to protect the eye from neurodegeneration induced by subconjunctival application of 6-hydroxydopamine (6-OHDA).', 'Intraocular neurodegeneration of anterior structures was induced by subconjunctival injections of 6-OHDA. Animals were pre-treated with topical corneal applications of Ap(4)A or saline.', '6-OHDA caused miosis, abnormal pupillary light reflexes, a precipitous drop in intraocular pressure and loss of VMAT2-labelled (vesicle monoamine transporter-2, a marker for sympathetic neurones) intraocular neurones. Pre-treatment with Ap(4)A prevented all of these changes from being induced by 6-OHDA, demonstrably preserving the sympathetic innervation of the ciliary processes. This neuroprotective action of Ap(4)A was not shared with the related compounds adenosine, ATP or diadenosine pentaphosphate. P2-receptor antagonists showed that the effects of Ap(4)A were mediated via a P2-receptor.'] Does diadenosine tetraphosphate protect sympathetic terminals from 6-hydroxydopamine-induced degeneration in the eye?