The pharmacokinetics of alpha-amino-p-hydroxybenzylpenicillin (amoxicillin) after intravenous, intramuscular and oral application were studied and the corresponding constants are reported. Following i.v. injection of 250, 500 and 1000 mg and infusion of 2 and 5 g the data are examined by means of a two- and three-compartment model, respectively. Calculations by adopting the two-compartment model reveal no significant dose dependency of the distribution constants and the serum concentration curves decline bioexponentially. A better fit of the regression curve to the experimentally determined serum concentrations is obtained by using the three-compartment model, indicating a slow and late disposition phase whose constants are significantly dose dependent. The share of this third phase in the total distribution of amoxicillin is 33% to 43%. After an i.m. injection of 250, 500 and 1000 mg amoxicillin is absorbed without any delay to 79% to 88%; the mean absorption constant is approximately 2 h-1. No significant dose dependency could be determined. Analogous results were obtained following oral administration of 250 and 500 mg. Absorption is delayed to 16 to 18 min; the mean absorption constant is 1.4 h-1 and 72% to 86% of the administered drug is absorbed without being significantly dose dependent. On the basis of the examined parameters the bioavailability of amoxicillin following extravascular administration can be regarded as reliable and safe.