Event-related potentials (ERPs) were recorded from squirrel monkeys (Saimiri sciureus) in a 90-10 visual "oddball" paradigm. A small, blue rectangle was presented every 2 s on 90% of the trials (background), whereas a yellow rectangle occurred on 10% on the trials (oddball). Electrical activity time-locked to these stimulus events was recorded from epidural electrodes before and following systemic administration of the alpha-2 noradrenergic agonist clonidine (0.1 mg/kg intramuscularly, IM). Baseline data in response to oddball stimuli showed a large, P3-like potential exhibiting a fronto-central maximum along midline electrodes and a parietal maximum along lateral electrodes. A frontally dominant, long-latency, negative slow wave (SN) consistently followed this P3-like potential. Amplitudes for P3 were larger following 10% than 50% probable oddball events. These results suggest that monkeys exhibit large, probability-sensitive P3-like potentials similar to the visual potentials reported in humans. Administration of clonidine had no effect on the amplitude, area, or latency of the monkey P3 component. This contrasts with our previous findings that the same dose of clonidine significantly decreases auditory P3s in these monkeys. Such differences may reflect distinct functional roles for norepinephrine in the processing of low-probability acoustic versus visual signals and argues against the hypothesis that norepinephrine is a common neurotransmitter substrate for auditory and visual P3-like potentials.