Previous research has shown that, in the context of event-related potential (ERP) prime-target experiments, processing meaningful stimuli such as words, phonemes, numbers, pictures of objects, and faces elicit negativities around 400 ms. However, there is little information on whether non-symbolic numerical magnitudes elicit this negative component. The present experiments recorded ERPs while adults made same/different judgments to serially presented prime-target pairs of non-symbolic numerical stimuli containing the same, close, or distant quantities. In Experiment 1, a negativity between 350 and 450 ms was elicited for targets preceded by primes of unequal quantity, and this was greater for close than for distant quantities. Change direction (decreasing or increasing) also modulated a similar negativity: a greater negativity was elicited by targets preceded by larger than by smaller quantities. Experiment 2 replicated the numerical distance and change direction effects for numerical judgments, but found no negative distance effect in a color comparison task when the same stimuli were used. Additionally, ERP effects of numerical distance were found under implicit conditions, and task proficiency in the number condition modulated implicit and explicit numerical distance ERP effects. These results suggest that the neural systems involved with processing numerical magnitudes contribute to the construction of meaningful, contextual representations, are partly automatic, and display marked individual differences.