All medications, except for the antipsychotic agents, will be kept stable throughout the protocol. These medications may include anticholinergics, mood stabilizers and antidepressants. After the screening phase lasting two weeks, patients will enter the cross-titration phase lasting three week. The cross titration schedule will be changed in accordance with Deutschman & Deutschman's 2005 recommendations. The current antipsychotic will be gradually decreased to zero and ziprasidone will be started at 40 mg bid po and raised up to 160 mg po bid during the cross-titration phase, according to clinical response and tolerance. After the cross-titration phase has concluded, the ziprasidone dose will range from 80 mg bid p.o. to 160 mg bid p.o. daily according to clinical response during the eight week treatment phase.