Our objective is to develop neural prostheses based on an array of microelectrodes implanted into the sacral spinal cord, that will allow persons with spinal cord injuries to regain control of their bladder and bowels. For our chronic cat model, we have developed two microelectrode arrays, one type containing nine discrete activated iridium microelectrodes and the second utilizing silicon substrate probes with multiple electrode sites on each probe. Both types can elicit an increase in the pressure within the urinary bladder of more than 40-mm Hg and/or relaxation of the urethral sphincter. A stimulus of 100 μA and 400 μs/ph at 20 Hz (charge-balanced pulses) was required to induce a large increase in bladder pressure or relaxation of the urethral sphincter. We found that 24 h of continuous stimulation with these parameters induced tissue injury (disrupted neuropil, infiltration of inflammatory cells, and loss of neurons close to the tip sites). However, a neural prosthesis that is intended to restore bladder control after spinal cord injury would not operate continuously. Thus, when this stimulus was applied for 24 h, at a 10% duty cycle (1 min of stimulation, then 9 min without stimulation) only minimal histologic changes were observed.