1. Permeabilization of cells mediated by P2X(7) receptors occurs to varied degrees in native and heterologous expression systems. Previous studies on P2X(7) receptors in parotid acinar cells suggested that ATP does not permeabilize these cells. 2. Modification of the assay conditions showed that ATP permeabilizes freshly dissociated rat parotid acinar cells to the fluorescent dye YOPRO-1. 3. The pharmacological and physiological properties of this effect indicate that permeabilization is mediated by the P2X(7) receptor. Adenosine 5'-triphosphate (ATP) and 3'-O-(4-benzoyl)benzoyl adenosine 5'-triphosphate (BzBzATP) were effective agonists with EC(50) values of 49.3 and 0.6 microM, respectively. 4. Permeabilization was best observed in low divalent cation concentrations and at physiological temperatures. Previous studies failed to detect permeabilization because of the sensitivity of this effect to temperature and divalent cations. 5. An important consideration in understanding the effect of divalent cations is that the fluorescence of YOPRO-1/nucleic acid complexes is directly quenched by addition of divalent cations. This must be considered if quantitative study of the interaction of divalent cations with P2X(7) receptors is carried out using fluorescent DNA-binding dyes. 6. In summary, our data show that P2X(7) receptors in parotid acinar cells can form large pores in the plasma membrane. This property likely contributes to signalling and may be cytotoxic and have particular significance in damaged or inflamed salivary glands.