The purpose of this study is to evaluate the efficacy of DNK333 compared to placebo for relieving symptoms of IBS-D in female patients.

Inclusion criteria:

• Women of 18 - 65 years of age with IBS-D as defined by the Rome II criteria.
• The Rome II criteria is a system used for diagnosing functional gastrointestinal disorders such as irritable bowel syndrome. It involves completing of a questionnaire about gastrointestinal symptoms.
• Patients who are 50 years of age and older must have had a colonoscopy OR a flexible sigmoidoscopy plus a double-contrast barium enema within the past 5 years, which demonstrated no clinically significant findings.
• Clinically significant findings may include but are not limited to malignant tumors, multiple (≥3) or advanced adenomas, inflammatory bowel disease, diverticulitis, ischaemic colitis, lymphocytic colitis, or collagenous colitis.
• Patients must report ≥ 3 days with IBS-related abdominal pain/discomfort plus at least 3 days of 2 or more of the following events during the baseline period:
• ≥ 3 bowel movements/day
• Bowel urgency
• Loose or watery stool

Exclusion Criteria:

• Patients who answer "yes" to either or both of the two weekly satisfaction questions during the baseline period. The questions are: (1) Over the past week did you have satisfactory relief of your IBS-related abdominal pain/discomfort? (2) Over the last week did you have satisfactory relief of your overall IBS-D symptoms?
• Patients with hard or lumpy stools for more than one day during the baseline period.
• Lactose intolerant patients relieved on a lactose free diet.
• Use of antidepressants (tricyclic, SSRI etc), opioid analgesic drugs or drugs specifically affecting bowel motility during the course of the trial.
• Women of child-bearing potential who do not use an acceptable methods of contraception.
• Pregnant or nursing (lactating) women.

Other protocol-defined inclusion/exclusion criteria may apply.