Solving simple arithmetic problems involves three stages: encoding the problem, retrieving or calculating the answer, and reporting the answer. This study compared the event-related potentials elicited by single-digit addition and multiplication problems to examine the relationship between encoding and retrieval/calculation stages. Results showed that the operation effect appeared as early as the encoding of the first operand and continued to the retrieval/calculation stage: compared to addition, multiplication elicited larger negative potentials in the left anterior electrodes and larger positive potentials in the right posterior electrodes. The consistency of this operation effect across the first two stages of arithmetic processing suggests that encoding of arithmetic problems can be modulated by the nature of representation of the to-be-retrieved arithmetic facts, and thus these two stages are additive rather than interactive. (Contains 1 table and 2 figures.)