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Platinum (Pt) is the most commonly used metal for stimulating electrodes. This study aims to determine the amount of charge that can be delivered without causing irreversible electrochemical reactions (charge injection capacity, Q _inj ) of Pt macroelectrodes (geometric surface area >0.001 cm ² ) in vitro and in vivo using voltage transient measurements. Pt macroelectrodes were stimulated with biphasic charge-balanced cathodic-first constant-current pulses in phosphate buffered saline. Potential excursions were measured (versus Ag/AgCl electrode) and used to determine Q _inj . The in vitro Q _inj were compared to those measured in vivo following: acute and chronic implantation close to the retina; chronic intracochlear implantation; and acute subdural implantation, in the cat. Q _inj increased with pulsewidth from 35 to 54 μC/cm ² for respective pulse widths of 100 to 3200 μs per phase in vitro. Qinj was significantly …