Pulmonary absorption of recombinant human granulocyte colony-stimulating factor (rhG-CSF) was examined in the rat. The relative bioavailabilities of rhG-CSF after intratracheal administration to intravenous and subcutaneous means were 11.6% and 27.4%, respectively, which were evaluated from the ratio of area under the curve (AUC) of the plasma rhG-CSF concentration versus time for 8h. The pharmacological availability after the pulmonary absorption was determined from the AUC for the increase ratio in total blood leukocyte numbers versus time curve. The avaiability was equal to or more than the availabilities after intravenous or subcutaneous administration. Comparison of the two different parenteral administration routes for rhG-CSF, intratracheal and intranasal showed the pharmacological availability after the intratracheal administration to be superior. The pulmonary absorption of rhG-CSF is thus an effective parenteral route of administration.