There are commercial benefits in being able to predict the time of ovulation, post weaning in the sow. These include the use of a single insemination, with associated decreased AI and labour costs, and increased litter sizes resulting from the correct timing of insemination (Waberski et al., 1994). To predict the time of ovulation in the sow, a variety of potentially influential factors have to be considered. Predictability may be increased by timed administration of gonadotrophin, which has been shown to reduce the variance in the weaning to oestrus interval in multiparous sows (Lancaster et al., 1985). This experiment was designed to determine the major factors influencing the weaning to ovulation interval and to examine the effects of exogenous gonadotrophin on the length and variation of this interval.