The lymphatic bioavailability (FL) of diazepam (DZ) and its major metabolite desmethyldiazepam (DDZ) was studied. DZ was administered in intravenous and intraduodenal boluses, and in intravenous infusion in three groups of rats with different total lipid (TL) content in the central lymph. The effect of a) different lipophilicity of DZ and DDZ, b) lymphatic TL content, and c) route of DZ administration on FL was determined. It was found that a) FL values of DZ exceeded the FL values of DDZ and b) FL values of DZ increased with increasing TL content in the lymph (an opposite relation was found in DDZ), and c) the highest FL value of DZ + DDZ sum after intravenous bolus administration was attained contrary to the lowest one after intraduodenal bolus administration.