To investigate the relationship among steroid hormones, estrous behaviors, and histological changes of sexual organs in breeding female blue foxes (Alopex lagopus), female individuals in different breeding stages were designated as three groups: typical estrus, atypical estrus, and non-estrus. Estrus identification, sample collection and hormone determination were performed. The hormone level was determined by radioimmunoassay (RIA) and XH6080 radioimmunoassay system was used in Xi'an Nuclear Instrument Factory. After the standard substance and samples were fully shaken with labeled antibody, they were then placed at room temperature for 15 min, centrifuged at 3 500 r/min for 15 min, and finally the supernatant was absorbed and discarded and the radioactivity count (CPM) of each precipitation tube was measured. Urine was collected and sent directly for hormone examination, and the contents of progesterone (P) and estradiol (E2) were determined. The ovaries and uterus were taken on the day of estrus and insemination. After dehydration, paraffin embedding, serial section and H.E staining, histological detection was conducted. Observation was conducted under optical microscope at 10 × 10 times lens and 10 × 100 times lens. The results showed that the female blue fox was in different phases of estrus, including vulva, vaginal resistance and keratinocyte stages, as shown in Fig. 1﹣3. The content of estradiol in the urine was significantly higher than that in the feces, especially in the typical estrus group, reaching 11 065.17 ± 546.76 ng/L, and the difference was significant. However, the content of progesterone in the urine was not significantly different from that in the feces, as shown in Table 2. The content of progesterone in the feces was slightly higher than that of urine in typical estrus females, but significantly higher than that of the urine in non-estrus females 16.61 ± 0.63 g/L, as shown in Table 1. Progesterone and estradiol contents were significantly correlated with estrus performance. When feces and urine were used as steroid test samples, although the hormone contents were different, the hormone changing trend was the same in these two kinds of samples. The ovary and uterus were relatively larger in female blue fox in typical estrus. Follicles at different stages and multiple corpus lutea could be observed in the ovary. The epithelial cells of uterine mucosa were columnar in shape and closely arranged, and a large number of glands could be observed in the lamina propria, as shown in Fig. 4 and 5. The ovarian and uterine development of the atypical estrus female was similar to that of the typical estrus female, as shown in Fig. 6 and 7. In the ovary and uterus of non-estrus female blue fox, the follicles in the ovary were mostly in an atresia state, without oocytes or corpus luteum. The lamina propria interstitial cells and myotrophic myocytes were arranged more closely, as shown in Fig. 8 and 9. It indicates that the blue fox's estrus state can be accurately determined by detecting the content of progesterone and estradiol hormones.