The present study is aimed to investigate several issues regarding the involvement of the frontotemporal brain language network in word production and the organization of the mental lexicon. Based on the data acquired by the functional MRI and using dynamic causal modeling (DCM), we checked whether inflectional processes could be divided into those relying on rules (for regular verb forms) and those involving only lexical memory (for irregular forms). As a result, the effective connectivity analysis revealed that the configuration of the investigated frontotemporal network varied as a function of the morphological process involved. Obtained data provided evidence towards the involvement of the left-lateralized frontotemporal system in the processes associated with the generation of regular forms. Based on these data it can be argued that the previously observed effect of increased psychophysiological interactions between the Broca’s area and the temporal gyri associated with the production of regular verbs supports the existence of rule-based regular form production. At the same time, the bilateral system is involved in the processes associated with retrieval from lexical memory.