1. The effects of reactive blue 2 and brilliant blue G, which have been shown to block extracellular ATP-evoked responses, were investigated to discover whether these compounds act as P2-purinoceptor antagonists in PC12 phaeochromocytoma cells. 2. Reactive blue 2 (10 to 100 microM) suppressed the ATP-stimulated dopamine secretion from PC12 cells in a dose-dependent manner. The concentration-response curve for ATP was shifted to the right and the maximal response was decreased by reactive blue (30 and 100 microM). Brilliant blue G (up to 100 microM) did not significantly affect the secretion. 3. Reactive blue 2 (10 to 100 microM) suppressed the ATP-activated inward current recorded from the voltage-clamped cells in a concentration-dependent manner. Brilliant blue G (up to 100 microM) did not affect the current. 4. The results suggest that reactive blue 2 but not brilliant blue G is a P2-purinoceptor antagonist in PC12 cells. The purinoceptors in these cells may be the same type as those involved in ATP-evoked smooth muscle relaxation, judging from the antagonism by reactive blue 2.