Inclusion Criteria:

1. Female patients from 18 to 65 years of age, inclusive.
2. History of IBS-d for at least 6 months prior to the initial screening visit.
3. Endoscopic/radiologic bowel evaluation to rule out cancer, obstruction, or other structural disease.
4. The patient must have completed at least six days of assessments in the patient diary in the one week period prior to the second screening visit.
5. Negative serum and urine pregnancy tests and post-partum for at least one year or not breast feeding at the initial screening visit and throughout the study. For patients able to bear children, an acceptable method of birth control must be used throughout the study. Patients unable to bear children must have documentation of such in the case report form (i.e., tubal ligation, hysterectomy, or post-menopausal [defined as a minimum of one year since the last menstrual period]).
6. Able to provide voluntary, written informed consent with comprehension of all aspects of the protocol.

Exclusion Criteria:

1. Serious underlying diseases, including psychiatric disorders or current history of conditions affecting bowel transit including biochemical or structural abnormalities, or GI surgery
2. Clinically significant abnormal examination findings or laboratory tests
3. Inability to stop taking certain medications, or a planned change in medication (including herbal remedies) which could interfere with study assessments
4. Use of drugs and or ethanol which may interfere with compliance of study procedures or influence study outcome
5. Presence of a medical condition which could interfere with the interpretation of study data
6. Significant use of nicotine or caffeine