This paper discusses the design of an application-specific integrated circuit (ASIC) suitable for mounting on a multi-electrode array for epidural spinal cord stimulation in rats. The ASIC acts as a demultiplexer, driving 12 electrodes on the array in any configuration. It is capable of routing biphasic constant current pulses of up to 1 mA to high impedance loads (with a maximum output voltage swing of approximately 25 V) and is small enough to be implanted into a rat's spinal column. Communication with its driver is achieved via 3 wires to minimize the number of interconnections. The circuit was implemented in a 0.18-μm high-voltage CMOS technology occupying a core area of 0.36 mm ². Power dissipation is about 110 μW. Post-layout simulations are presented which show the correct operation of the system.