The major mammalian sex hormones such as 17β-estradiol, androsterone, testosterone, estrone and progesterone, and non-steroidal estrogenic mimics are wide spread in the plant kingdom. Biosynthetic pathways of these hormonal substances in plants are partially known. Although the steroidal sex hormones have shown to enhance growth and reproduction of some plants in vitro, their roles in plant physiology is still illusive. Some of these compounds increase abiotic stress tolerance in plants and affect the activities of microorganisms including the peronosporomycete phytopathogens. The peronosporomycetes are fungus-like microorganisms, which are devastating pathogens of plants, animals, fishes, and humans. Mammalian estrogens and other estrogenic compounds display repellent activity against zoospores of the phytopathogenic pernosporomycete, Aphanomyces cochlioides. The estrogenic activities of known sex hormones and their repellent activity toward zoospores are likely to be correlated. Some peronosporomycetes also produce characteristic steroidal hormones for their mating for sexual reproduction. This report comprehensively reviews current updates on the occurrence of mammalian sex hormones in plants and discusses their roles in plants and plant-peronosporomycete interactions. Discovery of mating hormones and their functions in sexual reproduction of the peronopsoromycetes are also described.