The nasal absorption of non-glycosylated recombinant human granulocyte colony-stimulating factor produced by Esherichia coli (E. coli-rhG-CSF) was studied in rats. We investigated the effects of the basic conditions of the dosing solution and the enhancing effects of various substances to find means to improve the nasal absorption of E. coli-rhG-CSF. Relative bioavailability following the nasal administration of E.coli-rhG-CSF solution to subcutaneous administration was 3.6%. Osmotic pressure of the dosing solution had virtually no effect on the nasal absorption of E. coli-rhG-CSF. Absorption was enhanced by a decrease in pH, an increase in buffer concentration at pH2 and an increase in drug concentration. Sodium cholate, ascorbic acid, tartaric acid, beta-cyclodextrin and dimethyl-beta-cyclodextrin in remarkably increased the nasal absorption of E. coli-rhG-CSF. Condition adjustments to the dosing solution and the use of several enhancers were significantly concluded to increase the intranasal absorption of E. coli-rhG-CSF.