In vitro dissolution tests and biopharmaceutical investigations on healthy test persons were carried out with 4 different galenic tablet formulations. Three of these formulations were prepared by a special galenic process, the fourth corresponded to the usual tablet formulations. It was shown that the dissolution behaviour of the formulations in vitro, in accordance with the specifications of the American Pharmacopoeia (USP XIX), does not correlate with the parameters found in vivo. However, if the formulations in vitro are checked with the Sartorius model according to Stricker, correlations can be found with the in vivo behaviour of nitrofurantoin. A linear relationship was achieved by the special galenic formulation directly regulating the pharmacokinetic behaviour of nitrofurantoin. The known biological scatter of nitrofurantoin was strongly reduced by this process. The amount of the substance still present and circulating in the body creating the side effects could be reduced to a minimum. All facts considered, the therapeutic safety is also ensured.