<p>Functional magnetic resonance imaging (fMRI) was run while mental arithmetic tasks with three levels of difficulty were being solved. Problems involving subtraction, multiplication, and division at the easy level, elicited brain activity in the left inferior parietal lobule and the left angular gyrus, which may reflect the retrieval of information from long-term memory, and also, for division tasks, in the left inferior frontal gyrus, pointing to use of a procedural strategy. As task difficulty increased, brain activity in the left inferior parietal lobule and left angular gyrus became bilateral and more intense, and additional structures such as the superior frontal gyrus, supplementary motor area, inferior middle and temporal gyrus, and cerebellum became involved, indicating the need for increased neural resources for the solution of more complex problems. When solving three-digit division problems, bilateral activity was observed in the insular cortex, which is associated with all sorts of types of affective and cognitive processes. The areas of brain activation identified here highlight different systems involved in solving arithmetic problems of different levels of difficulty. Despite similarities in brain activation patterns, behavioral results showed statistically significant differences between arithmetic operations. This study contributes to the neural mapping of mathematical processes in healthy adults in stereotactic coordinates.</p>