The objectives of this study were to determine the effect of supplements containing 50% bypass protein with and without 10% fat on rebreeding performance of two year old range cows. Sixty-three black Angus x Hereford cows with nursing calves at the USDA Jornada Experimental Range were divided by calving date and sex of calf into one of three pastures. Cows received either a 20% crude protein supplement (20%S), 48 % crude protein supplement (which was 50% bypass protein) (48%S) or 48%S with 10% added fat (in the form of Alifet) (48%S+F) from calving until breeding (February to May). Cattle were weighed before calving, before exposure to bulls and again at weaning. Serum samples were collected twice before (10 d intervals) breeding to determine percentage of cows that had cycled prior initiating the breeding season.