Subjects having at least one condition selected from the group consisting of mental retardation, Down's syndrome, fragile X syndrome and autism are treated with a composition that includes gamma-am inobutyric acid agonists and/or MI muscarinic receptor antagonists. The gamma-am inobutyric acid agonist (GABA) can be a GABA(B) agonist, such as baclofen. GABA(B) agonists can be used in combination with Group I mGluR antagonists and MI muscarinic receptor antagonists in methods of treating humans.