1. Human calcitonin was administered into the distal colon and by intravenous infusion in eight healthy subjects in an open, fixed sequence, cross-over bioavailability study.
2. Intravenously infused human calcitonin elicited a standard pharmacokinetic profile in eight healthy subjects with a biphasic elimination with half-lives of 10.2 ± 0.7 min and 37.8 ± 2.5 min.
3. Colonoscopically administered human calcitonin was absorbed across the distal colonic mucosa in low amounts with a bioavailability of 0.00–0.22%.
4. Absorption from the distal colon was impeded by the presence of faecal material in three of the eight subjects.
5. We conclude that human calcitonin crosses the gastrointestinal epithelium of man. This may demonstrate the feasibility of an oral form for clinical use.