SUMMARY

A reasonably realistic physical model has been described for the simultaneous longitu-dinal spreading, fluid flow and absorption of drugs in solution under non-steady-state conditions in the small intestinal tract. Various input cases included first-order and zero-order stomach emptying and input from an infinite drug reservoir at constant infusion rate. The mathematical solutions were unique and rigorous. Theoretical simulations using reasonable physical parameter values illustrated the interrelationships of the longitudinal spreading diffusion coefficient, flow rate, apparent permeability coefficient and intestinal length on the change in concentration-distance profiles with time and the kinetics of appearance of unabsorbed rug at the end of the intestinal segment. The model is accessible to the design of intestinal absorption experiments and data interpretation a quantitative mechanistic basis and also provides the way for studying intestinal absorp-tion under more realistic situations