Commercially, restricting dietary nitrogen intake in breeding sows of lean genotype may be desirable. Reducing protein intake whilst maintaining high energy levels will result in limited lean tissue growth and promote fat accretion. This would be beneficial in ensuring more back fat reserves for lactation and reducing nitrogen pollution of the environment. However lowered protein levels during lactation have been shown to result in a negative nitrogen balance and extended weaning to oestrus intervals (King and Williams, 1984). In this experiment trans-rectal ultrasonography was used to assess any detrimental effects of protein restriction on ovarian activity in non lactating sows.