In two feeding experiments the retention of supplemental guanidine acetic acid (GAA) in broilers was investigated. In both experiments, the same three treatments were used; the basal feed was supplemented with 0, 0.6, or 6.0 g GAA per kg of feed. While in a growth study (experiment 1) day-old, male Ross 308 broilers were fed diets for 35 days, these diets were fed for only 8 days to fistulated broilers 34 days of age in a balance study (experiment 2). Feeding 0.6 g/kg GAA did not improve growth performance whereas 6.0 g/kg GAA resulted in a reduction of feed consumption and consequently of weight gain (P ≤ 0.05). Feed conversion was not affected and was 1.48 to 1.49 in all treatments. Increasing levels of dietary GAA gradually increased the creatine concentration in breast muscle and liver tissues (P ≤ 0.05) indicating a transformation and retention of dietary GAA as creatine. In experiment 2 the non-supplemented basal diet allowed us to determine the endogenous GAA, creatine, and creatinine excretions. Accordingly, only small amounts of these metabolites were recovered in feces while they were much higher in urine. Increasing dietary GAA intake increased fecal and renal GAA, creatine, and creatinine excretion and was significant (P ≤ 0.05) at 6.0 g/kg dietary GAA compared to no or 0.6 g/kg GAA supplementation. The mean true fecal digestibility of GAA (99%) was unaffected by the level of supplemental GAA. Considering renal GAA excretions, true availability of supplemental GAA was reduced with increasing dose (83% vs. 71%; P ≤ 0.05). Taking into account creatine and creatinine excretions above those of the basal diet, as they are a consequence of increasing dietary supply, true availability of supplemental GAA shrank from 76% (0.6 g/kg GAA) to 46% (6.0 g/kg GAA; P ≤ 0.05). Changes in blood creatine and creatinine levels reflected the changes observed in the liver and muscle tissues and may suggest increased transport to excretion organs. Data from these experiment were used to estimate the creatine requirement.