Increasing numbers of mares are competing at elite levels, and are recognised as being crucial to the production of high calibre sport horses. The management strategies utilised to look after horses affect their welfare and performance. By its very nature the domestication of horses has resulted in their confinement and also in major alterations to their natural social group structure. Most domestic pasture groups comprise mares and castrated males (Van Dierendonck, Sigorjonsdottir and Thorhallsdottir, 2002) and in the absence of a mature stallion are non-reproductive (Boyd and Kieper, 2002). Many equine behaviour studies have been directed at improving welfare and performance, but few centre on Personal Space Requirements (PSR). This study examines whether equine PSR varies between gender. Human personal space (PS) is defined as ‘the geographical component of interpersonal relations’ (Gifford, 1983) and its invasion is shown to cause instantaneous physiological stress (increased blood pressure and heart rate) and longer-lasting psychological stress (anxiety and tension), particularly when individuals can only endure exposure to stress passively (Sawada, 2003).