The hexacationic dye ruthenium red produce neuronal death in primary cultures. We injected messenger RNA (mRNA) from cultured neurons into Xenopus laevis oocytes to test whether this treatment can make oocytes sensitive to the damaging action of ruthenium red. Two-microelectrode voltage clamp and resting membrane potential were used to evaluate mRNA expression and to assess the effect of the dye on oocyte survival, when added to the medium or when injected into the cells, at 20, 50, or 100 microM concentrations. Injection of mRNA from cultured cortical or cerebellar granule neurons produced both new outward currents and membrane hyperpolarization. Exposure of mRNA-injected oocytes to extracellular ruthenium red for 24 h induced a remarkable depolarization, but no significant damage was observed. Injection of the dye into buffer-injected oocytes did not cause any change in membrane potential or cell survival, whereas in mRNA-injected oocytes an important depolarization was observed at 24 h after ruthenium red introduction, and 29% of the cells showed serious damage. The results suggest that oocytes become sensitive to intracellular ruthenium red toxicity because they express neuronal-specific proteins involved in cell death.