Objective To explore the effect of lactational TCDD exposure alone on growing development and cytochrome P4501A1(CYP1A1) in mice offspring. Methods Mature Kunming mice (clean animal,No.604017) were divided to 2 treatment groups (40 and 20 ?g /kg bw TCDD),2 vehicle controls and 1 normal control. There were 3 maternal mice and 25-28 pups in each group. Maternal mice were administered TCDD by intraperitoneal injection in three times on post-parturition days 1,3,5,and the mice offspring were exposed to TCDD by maternal milk. The changes of body weight and the development of reproductive system were observed. The pups were sacrificed on postnatal days (PND) 35,and CYP1A1 expression in the lungs of mice offspring were measured by immunohistochemistry. Results The average body weight of mice offspring in 2 TCDD treatments decreased significantly from PND14 (P