This study aims at investigating the production of the Greek modal and negative particles by non-fluent aphasic patients. These particles belong to the highest part of the verb periphrasis, so they are likely to be impaired in non-fluent aphasia, according to some hypotheses about agrammatic language. Moreover, there is an agreement relation between modality and negation, allowing the examination of agreement relationships in the clause domain. The data are compared to the predictions of recent theories on agrammatic language. The data provide evidence that modality is impaired in agrammatic aphasia, whilst agreement relations between clausal elements are spared. (Contains 9 notes, 10 tables, and 4 figures.)