Risperidone, widely used in the treatment of schizophrenia, has been shown to be effective in the treatment of manic and mixed episodes associated with bipolar disorders. Antipsychotic drugs like risperidone have also been used as additional therapeutic agents in the treatment of patients who are not responsive to mood stabilizers alone. This is a randomized, double-blind, placebo-controlled study to evaluate the effectiveness and safety of risperidone compared with placebo, as an addition to mood stabilizing drugs in the treatment of patients experiencing manic episodes. Treatment of one group of patients with haloperidol is used as an internal control in the trial. The study has two phases: a double-blind treatment phase (3 weeks) and an open-label phase (10 weeks). During the double-blind treatment phase patients receive risperidone, haloperidol, or placebo tablets to be taken once a day at gradually increasing doses (adjusted to 1 to 6 mg/day for risperidone and 2 to 12 mg/day for haloperidol), while continuing treatment with a mood stabilizer (lithium or valproate). In the open-label phase all patients receive risperidone with the dosage gradually adjusted to achieve optimal effectiveness (dose range of 0 to 6 mg/day); in this phase patients continue therapy with a mood stabilizer (lithium, valproate, or, for this phase only, carbamazepine). The primary measure of effectiveness is the change in the Young Mania Rating Scale (YMRS) total score from baseline to end of double-blind treatment. Additional assessments of effectiveness include the Brief Psychiatric Rating Scale (BPRS); the Clinical Global Impression (CGI), which evaluates the change in severity of the disorder; and the Hamilton Depression Rating Scale (HAMD). Safety assessments include the incidence of adverse events throughout the study; measurement of vital signs (pulse and blood pressure) and evaluation of the presence and severity of extrapyramidal symptoms by the Extrapyramidal Symptom Rating Scale (ESRS) at specified intervals; and clinical laboratory tests (hematology, biochemistry, urinalysis) before study initiation, at completion of double-blind treatment, and at the end of study. The study hypothesis is that daily treatment with risperidone as add-on therapy provides better effectiveness than placebo, as measured by Young Mania Rating Scale scores, in the treatment of the manic phase of bipolar disorder. Double-blind (daily doses, taken orally once a day) - Days 1 and 2: risperidone 2 mg, haloperidol 4 mg, or placebo. Days 3 and 4: risperidone 1 - 4 mg, haloperidol 2 - 8 mg, or placebo. Days 5 - 21: risperidone 1 - 6 mg, haloperidol 2 - 21 mg, or placebo. Open-label: risperidone 0 - 6 mg/day for 10 weeks.