The objective of this study, which formed part of a larger project, was to investigate the effect of feeding high levels of urea on the reproductive function of the lactating dairy cow. Increasing dietary protein intake can increase milk production, but may reduce reproductive performance (Laven and Drew 1999). McEvoy et al (1997), based on work which fed urea to sheep, suggested that this effect on fertility may be caused by ammonia. However, there is little information on the effect of feeding quickly degradable nitrogen (QDN) on the concentration of plasma ammonia in the dairy cow and the accurate measurement of plasma ammonia is difficult. The measurement of a more stable metabolite, such as urea, may be more useful, if it can be shown to be correlated with plasma ammonia.