The electrophysiological effects of the putative P2-purinoceptor antagonist reactive blue 2 (RB2) were investigated in strips of circular muscle of rat caecum with the sucrose gap technique. RB2 (0.1-1 mM) antagonized in a concentration-dependent manner, the amplitude, rate of rise and speed of onset of the inhibitory junction potentials elicited either with single pulse or with train of field stimulation at 10 Hz. A fully effective concentration of RB2 (0.5 mM) decreased the membrane response to high strength hyperpolarizing constant current pulses, thus indicating an increase in membrane conductance. At this concentration RB2 inhibited the hyperpolarization induced by the stable ATP analogue alpha,beta-methylene ATP, but did not significantly inhibit noradrenaline-induced hyperpolarization. RB2 (0.5 mM) also abolished the ability of carbachol to elicit spikes, while the carbachol-induced depolarization and the amplitude of accompanying mechanical responses were largely unaffected. The possible mechanisms responsible for the RB2-induced effects are discussed.