ATP is released by neurons and acts as a neurotransmitter andneuromodulator via purinergic receptors. Extracellular ATPconcentrations are precisely regulated by ecto-nucleosidetriphosphatediphosphohydrolases (ENTPDases). Resulting AMPis further metabolized extracellularly to adenosine, a potentinhibitory neuromodulator. We showed that two major subtypes ofectonucleotidases: ENTPDase 1 and 2 are expressed in the zebrafishretina both by westem blot and imrriunohistochemistry on 10 umtissue sections. Both enzymes are heterogeneously distributedamong retinallayers, principally expressed in the inner half oftheretina but their expression pattern overlaps only partially.ENTPDases 1 and 2 activities can be distinguished one from eachother by autoradiography using radioactive nucleotides as substrates.To date, we demonstrated extracellular ATPase activity on zebrafishretinal sections by this methodology. We are currently comparingwhether the site of ATP hydrolysis match es the immunoreactivitypatterns for both enzymes. In conclusion, the retina of zebrafishcontains two different ENTPDases with ATPase activity.lnterestingly, these enzymes show a specific distribution on cellsubtypes and synaptic retinallayers. Since ENTPDase 1 producesAMP in a much higher rate than ENTPDase 2, our results suggesta localized regulation of extracellular adenosine levels.