An intraindividual comparative single-dose study was carried out under carefully controlled conditions on 12 healthy volunteers in order to establish the bioavailability of trans-4-(2-amino-3,5-dibromobenzyl)-amino-cyclohexanol (ambroxol) the active principle of newly developed tablets and drops, in comparison to a commercial i.v. preparation. In an additional single-dose, cross-over study on 12 healthy volunteers the bioequivalence of ambroxol was investigated after administration of a newly developed vs. a commercial dose-equivalent, sustained-release dosage form. Following off-line derivatisation using formaldehyde to the corresponding tetrahydroquinazoline compound, ambroxol was assayed from plasma by high-performance liquid chromatography. A 2-compartment model was taken as a basis for the calculation of the plasma concentration curves and the pharmacokinetic parameters following intravenous injection of the drug. After i.v. administration, the terminal elimination half-life, the apparent volume of distribution and the total plasma clearance were determined to be 3.72 h, 1.52 l/kg and 565 ml/min, respectively. From the tablet and drop formulations the systemic availabilities were calculated to 73 and 81%, respectively; the mean transit times were determined to be 6.8 and 5.4 h, respectively. Both sustained-release dosage forms investigated are bioequivalent.