Although it is not a substantial constituent of dairy cattle feed, there has been much work on the effects of feeding supplemental fat on reproductive performance. Supplemental fat increases the energy concentration of the diet and may lessen the effects of the negative energy balance post calving. Adding fat has been reported to positively influence reproduction in a number of ways, for instance increasing plasma progesterone concentration and increasing the life span of the corpus luteum (for a review Staples et al., 1998). A number of studies have demonstrated the importance of progesterone in early pregnancy and in particular the timing and strength of the post ovulatory progesterone rise. The aim of this study was to establish the effect of a dietary supplement of fat, in the form of calcium soaps of fatty acids, on reproductive function and in particular on post ovulatory progesterone levels in lactating dairy cows.