Results from a series of recent experiments involving superovulated ewes demonstrate the important influence of nutritionally-induced alterations in preovulatory progesterone concentrations on the subsequent in vivo and in vitro development of their fertilized ova (McEvoy et al, 1993 and 1995; Creed et al, 1994). In essence, these show that high-plane feeding can suppress preovulatory progesterone concentrations to such an extent that the subsequent development of the ova is impaired both in vivo and during in vitro culture. An important practical question however remains unanswered in that no attempt has been made to study the effects of dietary energy concentrations, as opposed to plane-of-nutrition, on progesterone concentrations and ovum development. As a result, recommendations regarding which energy sources should be used as supplements to pasture around mating time are a matter of conjecture. Furthermore, in arid environments, roughage feeds are often in short supply and therefore command a much higher price per unit of energy than concentrate diets. Under these conditions it is not unusual to feed all-concentrate diets at mating, yet there are no published data for their effects on ovum development and embryo survival.