This report describe studies of the functioning of brain structures which are components of the frontotemporal system which is involved in the processes giving rise to speech and the organization of the mental lexicon. The studies address the question of whether it is possible, using functional tomography data, to discriminate the processes generating word forms into those carried out following rules (so-called regular forms) and those based on extracting forms from memory as whole units (so-called irregular forms). This was approached by carrying out experiments designed to identify how the origination of regular forms modulates the interaction between Broca’s area and two zones in the superior temporal gyrus of both hemispheres. It is suggested that when regular verbs are generated on the basis of symbolic rules (using a two-system approach), changes in interactions affect only the left hemisphere part of the system. Studies of cause effect relationships using dynamic causal modeling identified a relationship between the type of morphological process and the type of interaction between zones of the frontotemporal system. Thus, processes linked with generating regular forms, presuming construction of word forms from morphemes, are characterized by negative modulatory influences from the left zones of the superior temporal gyrus on activity in Broca’s area. These data support the view that the regularity effect seen in our previous studies of functional connectedness actually reflects the process of generation by rule and is supported by interactions between the left superior temporal cortex and Broca’s area. In addition, the generation of irregular verbs is characterized by an interaction between Broca’s area and the superior temporal gyrus in both hemispheres, which supports the suggestion that memory retrieval processes are involved.