A single case and group study methodology was adopted to investigate the cognitive mechanisms involved in propositional language and the underlying anatomical substrates. Patients with dynamic aphasia and patients with unselected focal frontal and posterior lesions have been investigated. Dynamic aphasia is characterised by a severe propositional language impairment despite well-preserved nominal language. The results obtained in three patients with dynamic aphasia (ANG, CH and KAS) suggest two functionally and anatomically distinct cognitive mechanisms. One set of cognitive mechanisms is responsible for high-level selection among competing verbal response options. This mechanism is specific to the language domain and is implemented by the left inferior frontal region. Evidence for this first mechanism comes from the dynamic aphasic patients ANG and CH and frontal patients with left inferior frontal gyrus lesions. These patients were severely impaired on word and sentence generation tasks only when a stimulus activated many competing verbal response options. By contrast, they were unimpaired when a stimulus activated a dominant response. The second set of cognitive mechanisms is responsible for generating a fluent sequence of novel thought. This mechanism encompasses novel verbal and non-verbal generation and is supported by bilateral frontal region. Evidence for this second set of mechanisms comes from the dynamic aphasic KAS and patients with frontal lesions. These patients were severely impaired in generating multiple connected sentences. These patients were also impaired in the voluntary generation of novel verbal and non-verbal responses. The convergence of findings from dynamic aphasia, patients with focal frontal lesions and neuroimaging are discussed. These data confirm a role of the frontal lobes in propositional language generation and specify at least two sets of cognitive mechanisms involved in this process.