The objective of the present experiments was to determine whether the levels of supplemental methionine sources currently used in practical diets exceed the capacity of the chick to convert the supplement to L-methionine. Supplemental sources examined included DL-2-hydroxy-4-(methylthio(butanoic acid (DL-HMB, Alimet, or MHA) and DL-methionine (DL-Met). Two approaches were taken: first, the amount of enzyme activity available for conversion of the two supplemental methionine sources was determined using optimum reaction conditions for each and chick liver homogenate as the enzyme source. These experiments showed that total liver enzyme conversion activity was 564 mumol/h for DL-HMB and 529 mumol/h for DL-Met. The total activities for the two sources were not different when measured at saturating substrate concentration. Second, to address the question of whether the enzyme is limiting for either source under practical feeding conditions, birds were fed starter diets supplemented with DL-HMB or DL-Met at .25% of the diet for 3 wk. When hepatic levels of free HMB and methionine were determined, birds fed DL-HMB contained 7.6 nmol HMB/g of liver and 84.7 nmol methionine/g of liver. Birds fed DL-Met had levels of 7.6 nmol HMB/g liver and 80.3 nmol methionine/g liver. These results indicate no accumulation of HMB or DL-Met, which might occur if conversion capacity were saturated. By calculation, a bird consuming 100 g/day of a diet supplemented with .25% activity would need to convert about 70 mumol/h, indicating a sevenfold excess of enzyme in the liver alone.