Objective To study the effects of psychological stress on breast development and abnormal levels of related hormones or their receptors in pregnant rats. Methods Pregnant Wistar rats were randomly divided into control group and experimental group. The experimental group rats were given unpredictable stress stimuli for 15 days, including noise, restraint, reversal of day and night, swimming in cold water and tail pinch-induced pain. Gross changes of the breast were observed, the diameter and height of breast nipple was measured, and breast coefficient was calculated. DNA and RNA levels in the breast tissues were determined by optical density measurement. The estrogen (E2), progesterone (P), growth hormone (GH) and prolactin (PRL) levels in plasma and breast tissues were determined by radioimmunoassay. Bmax and Kd of estradiol and progesterone receptors were measured by radolig and bindng assay. Morphological changes of the breast tissues were examined by light microscopy using HE staining. Results Compared with the control group, the nipple diameter and height, breast weight and breast coefficient were significantly decreased in the experimental group (P<0.05, P<0.01), the levels of DNA, RNA and RNA/DNA ratio in the breast tissue were significantly decreased (P<0.05, P<0.01), the levels of plasma E2, P and GH were significantly decreased (P<0.05, P<0.01), the levels of E2 and GH in the breast tissue homogenate were decreased also (P<0.05, P<0.01), the Bmaxs of E2 receptor and P receptor were decreased, but their Kds increased significantly (P<0.05, P<0.01), and the number of lobules and diameter of acini were significantly decreased (P<0.05). Conclusions Psychological stress can lead to breast dysplasia and abnormal levels of estrogen and progesterone and their receptors in pregnant rats.