This study sought to determine whether the incidence of perseveration in the responses of adult aphasics could be affected by altering stimulus factors such as semantic difficulty and the rate of presentation of stimuli. Thirty-one aphasic men supplied 1-word responses within three randomly arranged language tasks: sentence completion, picture naming, and word reading. Within each task subjects received stimuli from two 20-word lists of words contrasted for their frequency of occurrence in the language. Each list was presented once with 1-sec intervals between subjects' responses and subsequent stimuli, and once with 10-sec intervals. Results indicated that when semantic difficulty as measured by the frequency of occurrence of the single-word responses was reduced, the mean number of perseverations decreased significantly. When time intervals were increased from 1 to 10 seconds, perseverations decreased significantly. Sentence completion and picture naming elicited significantly more perseverations than word reading. Perseveration and other verbal errors were similarly affected by the variable of semantic difficulty, but only perseverative errors were significantly reduced when time intervals were increased. Word length, presentation order of tasks and conditions, and time postonset of aphasia appeared to have little relationship to perseveration.