In this thesis the influence of pneumo-vagina on the microbiological colonization of the genital tract and their manifestation in cytological smears was examined. For mares with poor vulval conformation a comparison of the bacterial growth before and after plastic surgery of the vulva and vestibulum was carried out, as well as the registration of conception rates after operation and insemination. The biggest reduction of the bacterial content in the reproductive tract was found between vestibulum and cranial section of the vagina. The increased number of contaminant bacteria in the cranial section of the vagina during abnormal vulval conformation could be reduced significantly by the plastic surgery of the vulva and vestibulum (p < 0.05). Assuming that an 18-20 days pregnant uterus is free from a colonization with virulent bacteria, a high significant reduction of the increased number of contaminant bacteria in the uterus during vulval incompetence was found after surgical correction (p < 0.01). In mares with poor conformation of the vulva, which had not been operated, the contamination of the reproductive tract had increased at the time of checkup in comparison to the first swab specimens, taken 18-20 days before. The share of mares with a negative bacteriological and positive cytological result was in case of vulval incompetence twice as large as if they had a proper vulval conformation. A decrease of these latent infections was found after plastic surgery of the vulva and vestibulum. From the operated mares a total of 72.9% became pregnant.