Biodistribution of125I-labeled 17α-vinyestradiol-3-acetate (125I-VE2A)in nude mice bearing human breast cancer containing different estrogen receptor (ER) content was studied to understand the relation between this compound and ER and, consequently, to develop the ER imaging. Each mouse was injected with 92.5 kBq tracer from tail vein and then killed after two hours. The radioactivity uptake rate in one gram of tumor tissue and tissues from other vital organs were measured, and the radioactivity uptake ratio of tumor to nontumor tissue was also measured. Results: The radioactivity uptake rate and the radioactivity uptake ratio of tumor to nontumor tissue in ER positive tumor (MCF-7) were much higher than those in ER negative tumor (MDA-MB-231). Conclusions: This compound, IVE2A has affinity to ER positive target organ or tumor and promise the probability to define the content and site of ER in vivo or in tumor.