We investigated the effects of ATP (adenosine 5Ј-triphosphate) and derivatives on leech neuropile glial cells, focusing on exposed glial cells. ATP dosedependently depolarized or hyperpolarized neuropile glial cells in situ as well as exposed neuropile glial cells. These potential shifts varied among cells and repetitive ATP application did not change their amplitude, duration or direction. In exposed neuropile glial cells, ATP most frequently induced a Na ϩ -dependent depolarization and decreased the input resistance. The agonist potency ATP Ͼ ADP (adenosine 5Ј-diphosphate) Ͼ AMP (adenosine 5Ј-monophosphate) Ͼ adenosine indicates that P2 purinoceptors mediate this depolarization. The P2Y agonist 2-methylthio-ATP mimicked the ATP-induced depolarization, whereas the P2Y antagonist PPADS (pyridoxal-phosphate-6-azophenyl-2Ј,4Јdisulphonic acid) reduced it. P2X agonists were without effect. Because the P1 antagonist 8-SPT (8-(p-sulphophenyl)-theophylline) also depressed ATP-induced depolarizations and some ATP-insensitive glial cells responded to adenosine, we suggest coexpression of metabotropic P2Y and P1 purinoceptors. The ATP-induced depolarization requires activation of Na ϩ channels or nonselective cation channels, whereas the ATP-induced hyperpolarization indicates activation of K ϩ channels. ATP also increased the intracellular Ca 2ϩ concentration ([Ca 2ϩ ] i ), that is independent of Ca 2ϩ influx but reflects intracellular Ca 2ϩ release possibly triggered by IP 3 formation. ADP and AMP also increased [Ca 2ϩ ] i , but were less efficient than ATP; adenosine and 2-methylthio-ATP did not affect [Ca 2ϩ ] i . In view of the mobilization of intracellular Ca 2ϩ , ATP is clearly different from other leech neurotransmitters, because it enables intracellular Ca 2ϩ signaling without causing prominent changes in glial membrane potential. Thus disturbance of the extracellular microenvironment and the demand for metabolic energy are minimized.