Abstract During whole-cell recordings from rat neostriatal neurons with Neurobiotin-filled patch-clamp electrodes, we observed markedly prolonged action potentials. Similar long-lasting action potentials were not detected when the tracer was omitted from the pipette solution. Resting membrane potential and input resistance remained unchanged in the presence of the tracer. The investigation of this effect revealed that Neurobiotin decreased the threshold for calcium spike generation probably by blocking a potassium conductance activated by depolarisation or by a direct action on calcium channels. The effect of Neurobiotin displayed a fast onset and was not observed during intracellular recordings using conventional microelectrodes.