I. To identify the maximum tolerated dose of imatinib (imatinib mesylate) in patients with recurrent oligodendroglioma and mixed oligoastrocytoma that are currently on enzyme inducing anticonvulsant therapy. (Study 1) II. To assess the efficacy of imatinib in patients with recurrent oligodendrogliomas and mixed oligoastrocytomas (with pathologic evidence of oligodendrogliomatous component) as measured by progression-free survival, response, and overall survival. (Study 2) III. To acquire pilot data on a patient group not traditionally eligible for recurrent oligodendroglioma and mixed oligoastrocytoma clinical trials (those having > 2 prior chemotherapy regimens or 2 prior chemotherapy regimens for recurrent/progressive disease). (Study 3) IV. To examine the toxicity and safety of imatinib in patients with recurrent oligodendrogliomas and mixed oligoastrocytomas (with pathologic evidence of oligodendrogliomatous component). (Studies 1, 2, and 3) V. To perform a preliminary correlative study of 1p/19q alterations, alpha platelet-derived growth factor receptor (PDFGR) gene amplification and levels of related downstream signaling elements in tumor tissue, with clinical study endpoints. (Studies 1, 2, and 3) VI. To perform a descriptive correlative analysis of steady state pharmacokinetic data regarding imatinib and active metabolites with the study endpoints. (Studies 1, 2, and 3)