Abstract: Objective To study the effect of L-arginine (L-arg) on the sebaceous gland of SD rat. Methods The SD rats of different sexes and ages were subjected to intragastric administration of L-arg at 10 mg·kg^-1·d^-1 (L-arg group) or saline (saline group) for 14 days.Blood samples were obtained from these rats for serum androgen determination and the pathological changes of the skin were observed. Results After L-arg treatment for 7 days,the dorsal hair of all the adult male rats appeared damp,greasy and yellow,while that of the adult and 5-week-old female rats looked normal.When the treatment was prolonged to 14 days,the same changes occurred in most of the adult female rats and all the 5-week-old female rats.The sebaceous glands of the abnormal skin showed hyperplasia in pathologic examination.The serum levels of androgen in rats with sebaceous gland hyperplasia were significantly higher than those in the control rats. Conclusion L-arg can promote sebaceous gland hyperplasia in rats in relation to increased androgen secretion induced by L-arg and NO.