The purpose of the study is to explore the effect of MPH on working memory, attention and decision making in adults with BD in remission or in depressed state. The results will be compared to findings in healthy adults and adults with ADHD. The investigators hypothesize that MPH will incur improved performance in all measures.

Following a detailed explanation about the trial and after obtaining informed consent from candidates, subjects will be asked to arrive at the cognitive laboratory at Shalvata MHC twice. Visits will include performance of computerized cognitive and decision making tasks, as well as two creativity tasks after the consumption of MPH or placebo.

Inclusion Criteria:

• Hebrew Speakers
• Age 21-50
• Diagnosis of BD I or II (296.7 and 296.89 classifications in the DSM-IV-TR), according to medical records. Diagnosis will be confirmed by a psychiatrist during screening process using the Structured Clinical Interview for DSM-IV-TR (SCID).

Exclusion Criteria:

• Diagnosis of a clinical disorder other than BD or ADD/ADHD which may impair performance on tasks used in the study.
• BD patients diagnosed with an acute manic episode at screening, as established by YMRS>8. It will also be required that 3 months have elapsed since the latest manic state, according to the patients' history.
• Participants for whom there exists a contra-indication for consuming Ritalin.
• Pregnancy or current breastfeeding; Female participants will be asked to report whether they are pregnant or currently nursing and if so, they will be excluded from the study.
• Drug use in the previous 6 weeks according to participants' self report
• Electroconvulsive therapy (ECT) in the previous 6 months