Abstract SINCE the hydroxy analogue of methionine (2-hydroxy, 4-methylthiobutyric acid) will support the growth of rats and chickens (Block and Jackson, 1932; Gordon and Sizer, 1954; Bird, 1952) the mechanism of its conversion to methionine was investigated. The utilization by the rat of alpha keto methionine and its conversion to methionine in the presence of suitable amino donors (Cahill and Rudolph, 1942; Meister and Tice, 1950; Meister et al., 1952 Meister, 1954) suggest that the keto acid is an intermediary between the hydroxy analogue and methionine. IN VIVO STUDIES Synthetic S35-labelled methionine hydroxy analogue (Blake and Wineman, 1956) and labelled methionine (Abbott Laboratories, Chicago, Illinois) were administered intravenously or encapsulated, admixed on a standard chick ration (“LC2 and LC2M,” Gordon and Sizer, 1955), to New Hampshire cockerels, 4–6 weeks old maintained in individual metabolism cages. Methionine was isolated from liver protein of the sacrificed chicks by the method of Weiss…