During the last fifteen years, colour Doppler sonography in women has led to an improvement in knowledge about physiological and pathological changes of the uterus and ovaries. In mares this technique has been used in the examination of the genital organs only since a few years. Therefore, in this article an introduction into the basics and technique of colour Doppler sonography is given. Then some of our findings using this technique are presented: changes in uterine blood flow depend on cycle stage of the mares. Furthermore we could show, that there are correlations between age, degree of fibrotic endometrial changes and uterine circulation in mares. Resistance to uterine blood flow increased with age and biopsyscore of the mares. The results received up to now using colour Doppler sonography demonstrate that this technique can provide additional information about uterine physiology and pathology in mares. Possibly, these findings lead to new concepts for the therapy of infertility.