Inclusion Criteria:

1. Male and non-pregnant, non breast-feeding female outpatients at least 18 years of age (no upper age limit).

2. History of constipation; the subject reports, on average, two or fewer spontaneous bowel movements per week that result in a feeling of complete evacuation and one or more of the following for at least 6 months before the selection visit:

   1. very hard (little balls) and/or hard stools at least a quarter of the stools
   2. sensation of incomplete evacuation following at least a quarter of the stools
   3. straining at defecation at least a quarter of the time. The above criteria are only applicable for spontaneous bowel movements, i.e., not preceded within a period of 24 hours by the intake of a laxative agent or by the use of an enema. Subjects who never have spontaneous bowel movements are considered to be constipated and are eligible for the trial.

Exclusion Criteria:

1. Subjects in whom constipation is thought to be drug-induced, or subjects using any disallowed medication.
2. Subjects suffering from endocrine disorders, metabolic disorders or neurologic disorders.
3. Subjects with a megacolon/megarectum or a diagnosis of pseudo-obstruction.
4. Constipation as a result of surgery.
5. Known or suspected organic disorders of the large bowel, i.e. obstruction, carcinoma, or inflammatory bowel disease.
6. Subjects with severe and clinically uncontrolled cardiovascular, liver, or lung disease, neurologic or psychiatric disorders (including active alcohol or drug abuse), cancer or AIDS, and other gastrointestinal or endocrine disorders.
7. Subjects with impaired renal function.
8. Subjects with clinically significant abnormalities of haematology, urinalysis, or blood chemistry.
9. Females of child-bearing potential without adequate contraceptive protection during the trial.