Restoring motor and/or sensory function for the hand and arm after spinal cord injury or limb loss may be aided by the use of high-count microelectrode arrays implanted in peripheral nerves. As proof of concept, we implanted 100-electrode Utah slanted Electrode Arrays (USEAs) intrafascicularly in median, ulnar, and/or radial nerves of anesthetized rhesus monkeys in acute preparations (n = 2),either at the elbow or just distal to the brachial plexus. We then examined the ability of individual electrodes to elicit motor responses in digits or wrist, or evoked potentials (EPs) in somatosensory or adjacent motor cortex. Muscle activation was monitored via evoked myoelectric responses recorded from 16-22 sets of wire pairs implanted in forearm muscles, extrinsic hand muscles, or intrinsic hand muscles, and by visual observation and video recording. Cortical EPs were monitored via recordings from 12 skull screws. Multiple different implanted electrodes evoked varied motor and sensory responses. For example, 79 individual electrodes of one USEA implanted in median nerve near the elbow evoked specific wrist or digit movements (presumably, some of the remaining electrodes were in extrafascicular nerve tissue). Threshold maps of motor responses evoked by the different electrodes of the USEA indicated that motor fibers were somatotopically organized within the nerves. Input-output curves generated by varying stimulus pulse duration further indicated that muscle activation could be quite selective, recruiting full or nearly full activation of a particular muscle with little or no recruitment of other muscles. Similarly, stimulus trains (~30-50 Hz, ~1/2 s) delivered to individual electrodes evoked different types of specific digit or wrist movements. USEA stimulation also elicited cortical EPs of short (~3 ms) latency that grew systematically with increasing stimulus strength and were spatially localized, suggesting direct activation of sensory nerve fibers. These results indicate that stimulation via USEAs implanted in peripheral nerves could be used to restore hand and wrist motor function in paralyzed individuals, or cutaneous and proprioceptive sensation in individuals with limb loss.