Inclusion Criteria:

Subjects who meet all of the following criteria are eligible to participate in this trial:

1. Males or females, inpatients or outpatients, aged 17 to 70 years inclusive.
2. Diagnosis of Bipolar II Disorder, current episode depressed, without psychotic features. Specifically, patients must have experienced at least one previous episode of hypomania lasting at least 2 days, and no previous manic episodes.
3. The current episode of depression has a duration of at least 6 weeks.
4. Montgomery-Asberg Depression Rating Scale score of at least 18.
5. If female and of child-bearing age, must be using a reliable method of birth control. Reliable methods of birth control include: oral contraceptive pill or patch or surgically implanted device; intra-uterine device (IUD); tubal ligation; barrier device such as diaphragm or condom plus spermicidal jelly or foam; or abstinence.

Exclusion Criteria:

Subjects meeting any of the following criteria are not eligible to participate in the trial:

1. Manic or hypomanic symptoms, defined as a YMRS score of 16 or greater.
2. Treatment with ECT or a depot antipsychotic medication within eight weeks prior to enrolment; or treatment with an experimental drug within 30 days prior to enrolment.
3. Known lack of response to, or intolerance for, Lamotrigine. Lack of response is defined as failure of depressive symptoms to improve after a trial of an acceptable dose of medication, ie. 100 mg daily or greater of Lamotrigine for at least four weeks.
4. Depressive symptoms secondary to substance use or a general medical condition, in the opinion of the investigator.
5. Diagnosis of an anxiety disorder, including Generalized Anxiety Disorder, Social Anxiety Disorder, Panic Disorder, Agoraphobia, Obsessive Compulsive Disorder, Specific Phobia, Post-Traumatic Stress Disorder, or Acute Stress Disorder, which was the primary focus of clinical attention in the year preceding enrolment.
6. Diagnosis of Schizophrenia, Schizoaffective Disorder, or Delusional Disorder.
7. Substance dependence within one month of enrolment, except for dependence in full remission, and except for caffeine or nicotine dependence, as defined by the DSM-IV-TR.
8. Diagnosis of Borderline Personality Disorder, Narcissistic Personality Disorder, Histrionic Personality Disorder, or Antisocial Personality Disorder, which was the primary focus of clinical attention in the year preceding enrolment.
9. Significant risk of harm to self or others, in the opinion of the investigator.
10. Use of any cytochrome P450 inducer or inhibitor within five half-lives prior to enrolment.
11. Pregnancy or lactation in female subjects.
12. Unstable or inadequately treated medical illness, as judged by the investigator.
13. Liver function tests (AST and ALT) three times the upper limit of normal.
14. Glomerular Filtration Rate (GFR) of less than 60 mL/min per 1.73m2
15. A history of significant cardiac conduction abnormalities, as determined by the investigator.