Abstract: An experiment was made to study the effect of the duration of the excreta collection period on the True Metabolizable Energy (TME) values of cereals as wheat, maize and barley and wheat bran. The experiment was performed with 48 mature Cornish and Rhode Island Red (RIR) cockerel strains, under standard conditions. Droppings voided during the experimental period were collected and assayed for gross energy and nitrogen. The results showed that the rate of excretion of EEL (32.71% for Cornish and 17.82% for RIR) and nitrogen (40.26% for Cornish and 22.80% for RIR) voided by adult birds of two strains decreases during the period of fasting bird (12 to 72 h). The effect of fasting period (12, 24, 48 and 72 h) on the TME values obtained of barley, wheat and maize 18.98, 6.24 and 16.32% for Cornish and 12.78, 7.82 and 8.31% for RIR birds, were significantly due to be reduced respectively. The finding of this study proves those 48 h fasting birds of two strains are suitable for routine of excreta collection time on estimates of metabolic plus endogenous energy losses for true metabolisable energy values for cereals such as wheat, wheat bran, maize and barley.