1. The nature of the galactokinase present in the livers of foetal, newborn and adult rats was examined by the application of several separation procedures and by measurement of a range of kinetic parameters. 2. No evidence of enzyme heterogeneity at any stage of development was found during gel filtration on Sephadex G-100, column chromatography on DEAE-cellulose or a variety of electrophoretic procedures. 3. The Km values, inhibition characteristics and other kinetic parameters appear to remain constant during development. 4. Rat liver galactokinase activity does not adapt to dietary changes in either the adult or the newborn rat; hence it is unlikely that the presence of galactose in milk controls the enzymic activity profile during development. 5. On the present evidence it is concluded that only one form of galactokinase is present in rat liver and that the enzymic activity is controlled by non-dietary factors.
