The single channel properties of four P2X receptors heterologously expressed in Chinese hamster ovary cells were determined from outside-out patch recordings. P2X1 and P2X4 channels opened in brief flickery bursts with conductances of approximately 18 and 9 pS respectively. P2X2 receptors had a single channel conductance of approximately 21 pS. Openings of P2X3 channels were too rapid to be reliably resolved. Application of ATP to macro-patches expressing multiple P2X2 or P2X4 receptors evoked sustained currents, in contrast responses of P2X1 or P2X3 receptors desensitised during continued agonist application. These data show that four different homomeric P2X receptors form ion channels with distinct single channel properties.