The bioavailability of doxycycline (Doxy-Diolan 100 tablets, test, active substances: 100 mg doxycyclin per tablet) was compared with that of another commercially available tablet-formulation containing the same active substance (reference). In a cross-over study, 16 young healthy male volunteers were administered in fasting state orally by one tablet containing 100 mg active substance. The concentrations of doxycycline were determined in plasma and saliva by a high-performance liquid chromatographic assay. Mean maximum plasma concentration (cmax +/- standard deviation) of doxycycline were 1.57 +/- 0.40 micrograms/ml (test) and 1.59 +/- 0.38 micrograms/ml (reference), respectively, and were reached 1.47 +/- 0.55 h and 1.66 +/- 0.57 h after administration. Plasma half-lives were 16.6 +/- 2.9 h and 16.8 +/- 3.0 h, the areas under the plasma concentration-time curves (AUC0-00) 29.3 +/- 4.5 mg/l.h and 29.7 +/- 4.4 mg/l.h. The concentration of doxycycline in saliva were low, median maximum concentrations of 50 ng/ml were measured 1-2 h after administration. The statistical evaluation revealed bioequivalence between both drugs.