Two experiments are reported which show that extraverts experience significantly stronger McCollough Effects than introverts. In both experiments the strength of the McCollough Effect (ME) was measured by the match-interference method devised by Shute (1979). The technique was found to be predictably sensitive to the eye tested and to exposure of the non-adapted eye. In the first experiment monocular ME strength was measured at 12-min intervals over two hours in four conditions and found to conform to a power function. Sixteen extraverts showed significantly (p < .00005) stronger initial ME strength than 21 introverts and in both groups the effect ceased to be apparent within about two hours. Log-log plots of decrement from initial strength indicated no significant extraversion differences in decay rates, with a function gradient of about 1/2. In a second experiment six extraverts and nine introverts from the original group were retested with binocular presentation with a similar outcome. These findings support Shute's hypothesis that introverts would show weaker MEs than introverts but do not support his hypothesis that introverts' MEs would decay more quickly. They offer some indirect support for Shute's proposal that the ME may be an indicator of central cholinergic activity in man.