1. The aim of this study was to functionally characterize the recombinant mouse P2X(4) receptor and to compare its pharmacological properties with those of the human and rat orthologues. 2. Whole cell recordings were made from rafts of HEK-293 cells stably expressing recombinant mouse, rat or human P2X(4) receptors, using Cs-aspartate containing electrodes (3 - 8 MOmega) in a HEPES-buffered extracellular medium. 3. The agonist potency of ATP at the three species orthologues was similar, with mean EC(50) values of 2.3 microM, 1.4 microM and 5.5 microM, respectively. 4. Adenosine-5'-tetraphosphate (AP4) acted as a partial agonist with respect to ATP at the mouse and human P2X(4) receptors (EC(50)=2.6 and 3.0 microM), but was significantly less potent at the rat orthologue (EC(50)=20.0 microM). alpha,beta-methylene adenosine-5'-triphosphate (alpha,beta-meATP) also acted as a partial agonist, producing 29% of the maximum response at the mouse P2X(4) and 24% at the human P2X(4) receptor. 5. In contrast to the other species orthologues, alpha,beta-meATP failed to elicit a significant agonist response at rat P2X(4) receptors, and was found to act as an antagonist, with an IC(50) of 4.6 microM, against 10 microM ATP. 6. Mouse P2X(4) receptors were found to be sensitive to the antagonist, pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) (IC(50)=10.5 microM), as were human P2X(4) receptors (IC(50)=9.6 microM). The rat receptor however, showed a low sensitivity to PPADS (IC(50)>100 microM). 7. All three orthologues were relatively suramin-insensitive (IC(50)>100 microM) and insensitive to 1-[N, O-Bis(5-isoquinoline sulphonyl)benzyl]-2-(4-phenylpiperazine)ethyl]-5-isoquinoline sulphonamide (KN-62; IC(50)>3 microM). 8. Our results suggest that the pharmacological properties of the mouse receptor are most similar to the human P2X(4) receptor, and differ markedly from the rat receptor.