The most important factor affecting the overall efficiency and profitability of pig production is the accurate timing of insemination in the breeding herd. Low conception rates and litter sizes result from pigs inseminated during the ‘incorrect’ period of oestrus in relation to ovulation. The average time at which ovulation takes place is between 38 and 42 hours after the onset of oestrus (Du Mesnil du Buisson, Mauleon, Locatelli and Mariana, 1970) with highest fertility achieved for matings carried out approximately 12 hours prior to ovulation (Dziuk, 1970). The back pressure test in the presence of a boar is generally used to indicate oestrus but does not indicate the time of onset or the stage of oestrus.