Humans are able to flexibly change patterns of thought or internal "cognitive sets" to find adequate solutions when faced with problems. Cognitive flexibility underlies the ability to act adaptively in changing environments. Accumulating evidence from functional magnetic resonance imaging (fMRI) investigations in humans suggests that functional networks of prefrontal and posterior parietal cortices (PPC) may be critical for such cognitive flexibility. To assess the single-cell level dynamics underlying these cortical processes, we recorded neuronal activity from the PPC of monkeys performing an analog of the Wisconsin Card Sorting Test (WCST). The WCST was originally devised to probe cognitive flexibility in humans. We modified the test for monkeys, providing a task requiring flexible shifting between color-matching and shape-matching behavior. The results revealed a group of PPC neurons whose activity signaled the direction in which monkeys would shift their cognitive set, i.e., from shape to color or from color to shape. These neurons were activated in preparatory processes preceding the actual behavioral output by about 4 sec, and the firing significantly predicted the success/failure of monkeys´ set shifting. Our analyses further demonstrated that another neuronal group was involved in transforming such preparatory processes into behavioral output (i.e., execution processes), by exhibiting shift-selective activity modulation both in preparation and execution processes. These cells still exhibited activity when the monkeys spontaneously performed set shifting without any external cues, which may reflect set-shifting mechanisms that can be driven by either internal or external triggers. Our results suggest that distinct neuronal groups are dynamically recruited for the different subprocesses involved in cognitive set shifting.