Long term experiments were carried out to determine the influence of varying energy intakes on the levels of blood components in lactating beef cows. Lowering the dietary energy intake to 0-95x maintenance produced a fall in bodyweight which continued throughout a 10 week feeding period. There was an initial fall in the plasma glucose level. After four weeks the plasma glucose level began to rise, and at the end of the 10 week period it had almost reached its original level. When bodyweight and plasma glucose were falling cows were infertile, when bodyweight was falling and plasma glucose was rising cows were fertile.