The aim of the present study was to evaluate the soft corticosteroid BNP-166 in rats and dogs treated orally with 0.2, 2.0, and 20.0 mg/kg for 28 days and the reversibility of any abnormalities during a 14-day post-dosing period. The test substance, BNP-166, was well tolerated during the 28-day treatment period. The observed changes were all characteristic for the pharmacological actions of a glucocorticoid. Treatment related changes occurred in the adrenals and thymus, and, to a lesser extent, in the lymph nodes, spleen and liver. There were no statistically significant reductions in the cortisol levels of all groups in the 0.2 and 2 mg/kg treatments. Significant reductions were observed in the high-dose group (20 mg/kg), but levels returned to normal by the end of the 14-day recovery period. Based on the results, the No Observable Adverse Effect Level (NOAEL) of BNP-166 soft corticosteroid in rat and dog after 28-day oral administration is 2 mg/kg. This value is approximately 40 times higher than that of budesonide. Pharmacodynamic and receptor binding studies have shown BNP-166 to have a similar potency to budesonide; therefore, BNP-166 can be considered safer when administered orally than other corticosteroids such as prednisolone or budesonide.