1 CHO-KI cells were examined for their cellular responses to the P2 receptor agonist, 2'- and 3'-O-(4-benzoylbenzoyl)-ATP (DbATP), and for the presence of mRNA for P2X receptors. 2 Reverse transcriptase-polymerase chain reactions, using primers directed against the rat P2X subunits, detected the presence of P2X, but not P2X(1)-P2X(6) subunits. 3 DbATP (EC(50 similar to)100 mu M) evoked non-desensitizing inward currents which reversed at similar to 0mV, suggesting activation of a non-selective cation channel. ATP also evoked inward currents but was less potent than DbATP. 4 DbATP also stimulated the accumulation of (45)calcium (Ca-45(2+)) and the DNA binding dye, YO-PRO-1, in CHO-K1 cells. Both responses were inhibited by NaCl and MgCl2. In 280 mM sucrose buffer, 45Ca(2+) accumulation was measurable within 10-20 s of agonist addition, whereas YO-PRO-1 accumulation was only detectable after 8 min. ATP and ATP gamma S were also agonists but were less potent than DbATP, while UTP, 2-methylthio ATP, ADP and alpha beta methylene ATP were inactive at concentrations up to 100 mu M. 5 DbATP increased lactate dehydrogenase release from CHO-K1 cells, suggesting cell lysis, although this effect was only pronounced after 60-90 min. 6 These data suggest that CHO-K1 cells express an endogenous P2X(7) receptor which can be activated by DbATP to cause a rapid inward current and accumulation of Ca-45(2+). Prolonged receptor activation results in a delayed, increased permeability to larger molecules such as YO-PRO-1 and ultimately leads to cell lysis. Importantly, the presence of an endogenous P2X(7) receptor should be considered when these cells are used to study recombinant P2X receptors.