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Although it is established that developmental methylmercury exposure produces severe motor and sensory impairment, the effect on cognitive function is less clear. To explore this issue, monkeys with robust methylmercury-induced deficits in visual, auditory, and somatosensory function were tested on a series of tasks assessing central processing speed, which is highly correlated with intelligence in humans. Five monkeys (Macaca fascicularis) were dosed from birth to 7 years of age with 50 micrograms/kg/day of mercury as methylmercuric chloride. Blood mercury levels were stable at 0.8-1.1 micrograms/g until cessation of dosing. When they were 20 years old, these monkeys and four age- and rearing-matched controls were tested on a series of simple and complex reaction time tasks. The monkey sat in a primate chair with a stainless steel bar centered at waist height. Four push buttons equidistant from the steel bar were mounted on a vertical Plexiglas panel in front of the monkey. The monkey was required to make contact with the bar, then release the bar and push the appropriate button in response to a change in stimulus conditions. For the first task (simple reaction time), the monkey was required to respond on a button when it changed from unlit to red. The monkey then performed a sequence of complex reaction time tasks: two-button, four-button, and several tasks of increasing complexity using four buttons and multiple colors. For each task, the latency to release the bar after the stimulus change (central processing speed) and to move the hand from the bar to the button (motor speed) were determined. Lastly, the monkey was required to make the quickest possible motor response on the simple reaction time task. There were no differences between groups on any aspect of the experiment. These data provide further evidence for absence of cognitive impairment in monkeys exposed developmentally to methylmercury.