For the evaluation of the bioavailability of drugs administered through a minimal jejunostomy catheter (MJC) postoperatively, ampicillin and metronidazol were administered via this route. The study with ampicillin consisted of a first cross-over experimental trial, in which six dogs were used, and a second based on the experience with four human volunteers. As a measure of the absorption of the drug, the serum levels of the lower plasma area are monitored. The statistical analysis was done by means of logarithmic transformation of the areas, and two-way analysis of variance. The posterior distribution of the relative bioavailability of the drug was obtained by applying a bayesian method. The results showed an important decrease of the ampicillin absorption, while the expected bioavailability when administered through MJC was 0.24, and the 95% interval (0.19, 0.30). The expected bioavailability of metronidazol when administered through MJC was 0.84, and the 95% interval (0.66, 1.07). This new route of drug administration in patients who undergo abdominal surgery, is shown to be of great importance from a clinical point of view, and should be kept in mind.