Three monkeys learned a delayed conditional Go/No-go discrimination task. The sequential events of the task were the instructional color stimulus presentation (S1), the first delay (D1), discriminative pattern stimulus presentation (S2), the second delay (D2), and the response (Go or No-go). Single units in the principal sulcus, arcuate area and the inferior convexity of the prefrontal cortex were studied during this task. Of 512 task-related units recorded, 29% had stimulus-related differential activity. Type I units (n = 46) responded differentially to the two S1 stimuli. Type II units (n = 7) responded differentially to the two S2 patterns. Type III units (n = 65) also responded following the S2 presentation, but their activity depended upon whether Go or No-go responses were being signaled, rather than the particular S2 pattern. Type IV units (n = 54) appeared to code both the stimulus properties and behavioral significance of S2. A few premotor units (Area 6) were also examined; they showed either type III (n = 12) or type IV (n = 2) activity. In summary, these results indicate that primate prefrontal cortex participates in visual information processing and may code several aspects of visual stimuli including behavioral significance and mnemonic representations.