The present study was a variation of the number reduction task of Woltz et al., 2000 and was designed to get new insights into the differences of implicit and explicit learning processes. The experiment consisted of a serial arithmetic problem structured in a special manner (ABCCB) which allowed a strategy change. The subjects had to respond to the numbers presented on the screen depending on specific rules. The participants were alluded to the possibility to shorten their key entries if they recognize the structure of the task. However, this instruction was formulated implicitly. During the task ERPs were recorded. The subjects were subdivided into two groups according to whether they discovered (explicit group, EG) or not (implicit group, IG) the rule. The hypothesis was that explicit learning and implicit learning is reflected in differences in the ERPs before strategy change. Twenty-six subjects participated in the experiment. Six of them were assigned to the EG and 20 to the IG. The EG showed a significant larger P2 and P3 to the first number of the series compared to the IG (p = 0.011 and p = 0.003, respectively). The results indicate that subjects differ in the ERPs before they are aware of the regularity. One can suppose that explicit learners are not only more attentive than implicit learners but also additional modules are activated.