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Previous fMRI studies have demonstrated that older adults who perform as well as young adults on certain cognitive tasks recruit additional brain regions relative to younger adults while performing these tasks. This phenomenon has been interpreted as a compensatory response and may reflect an effort to maintain performance in the face of increasing changes in cognitive difficulty or age-related brain changes in structure and/or function. Whether the compensatory response is specific to older adults or represents a more general response of any individual to increasing task difficulty is unclear. The present fMRI experiment explored age differences in brain activity associated with increases in task difficulty in two tasks, an episodic-retrieval task that is expected to be difficult for older compared to young adults, and a lexical-semantic task that is expected to be more difficult for young compared to older adults. In the lexical-semantic task, participants judged whether pairs of words were synonyms or antonyms. In the episodic task that followed, participants made yes/no memory judgments for the word pairs previously presented. Difficulty was manipulated using word frequency -low frequency words are more difficult in the lexical-semantic judgment task and easier in the episodic task. Young (ages 18-24) and older healthy adults (ages 60-83) were scanned on a 3T GE magnet using a single-shot spiral pulse sequence. Behavioral results showed a double dissociation – older adults were adversely affected by word frequency in the episodic but not the semantic task, while young adults were adversely affected by word frequency in the semantic but not the episodic task. Both groups showed activation in similar task-related and task-general regions regardless of difficulty level). Age-related differences were observed for task-specific and linear increases due to difficulty. Linear increases in fMRI activation were associated with younger adults showing increased task diffic