Abstract: Objective To identify the effect of milk from pregnant cows on urinary estriol and progesterone in young female rats.Methods 21-day-old female SD rats were randomly divided into milk-1 group,milk-2 group and control group with 12 animals for each group.The rats in milk-1 group,milk-2 group and control group were fed milk from pregnant cows,commercial milk and assorted feeds,respectively.All rats had free access to the rat feeds and water after fed milk or assorted feeds every day.Then 24 hour urine of each rat was collected after 10 days.The urinary estriol and progesterone were detected by the immunochemiluminometric assays(ICMA).Results The amounts of urinary estriol(ng/24h)of the rats in milk-1 group,milk-2 group and control group were(6.084±1.308)ng,(6.074±1.466)ng and(3.447±1.149)ng,respectively,and that amounts in the two milk group were both significantly higher than that in control group(P<0.01);The amounts of urinary progesterone(ng/24h)of the rats in milk-1 group,milk-2 group and control group were(469.791± 258.258)ng,(434.361±167.606)ng and(221.296±159.153)ng,respectively,and that amounts in the two milk group were both significantly higher than that in control group(P<0.05).But neither estriol nor progesterone in the rats between the two milk groups had significant difference(P>0.05).Conclusion The milk from pregnant cows or commercial milk in this test could both affect the steroid metabolism in young female rats.And the effect on young female rat between two milk groups didn'thave significant difference,which indicated that the commercial milk containsed a considerable quantity of the milk from pregnant cows.