ATP-induced Ca2+ transients were examined in individual PC12 cells of a well defined clone, before and after treatment with nerve growth factor (NGF) to induce a neurone-like phenotype. Using reverse transcriptase PCR these cells were found to express mRNA for several P2 receptors. In undifferentiated cells the ATP-induced Ca2+ response was entirely dependent on Ca2+ influx, could not be mimicked by UTP, alpha,beta-methylene ATP or dibenzoyl ATP or be blocked by pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS). ATP had no significant effect on levels of cyclic AMP or inositol 1,4,5-trisphosphate (InsP3). These results suggest that in undifferentiated PC12 cells ATP mainly acts on a P2X receptor, possibly the P2X4 subtype. After treatment with NGF for 7 days the ATP response was increased and partially sensitive to PPADS. A component of the ATP-induced Ca2+ increase was due to mobilisation of intracellular Ca2+ stores and another to capacitative Ca2+ entry. UTP caused an increase in intracellular Ca2+, and InsP3 formation could be stimulated by ATP and UTP. ATP also caused a small increase in cyclic AMP, but this was abolished in the presence of indomethacin. Thus, after NGF treatment ATP acts partially via a P2Y receptor, possibly the P2Y2 subtype.