It was established earlier that the maintenance of rats on a galactose-rich diet induced in rat liver a sequental induction of enzymes, converting galactose to glucose (galactokinase, galactoso-1-phosphaturidytransferase and uridyndiphosphogalactose-4-epimerase); this was followed by the repression of these enzymes. Against the background of the enzyme repression, the continuation of galactose treatment leads to the development of galactosemia symptoms; cataracts, liver lesions growth retardation. Animals with the increased susceptibility to galactose were found in population of Wistar rats; in these animals rapidly developing enzyme induction is followed by sharp repression of enzymes of the galactose metabolism and in them cataracts appear 17-19 days after the start of feeding a galactose-rich diet. A part of the population is resistant to the galactosemic effect of galactose and in these animals cataracts develope only 40-44 days after the beginning of the galactose feeding. By inbreeding of individuals extremely susceptible to galactose and those resistant to it, new substrains of rats were obtained. It is found that in the rats of the galactose-susceptible substrain a number of galactosemic features develope spontaneously and that these features are inheritable. Thus, 85% of the animals of the age of 2.5-6 months have cataract, lens opacities and other lens impairments. In the galactose-resistant substrain no cataracts or lens opacities develope and only slight changes of the lens are observed in 15% of the animals. In the susceptible substrain other features characteristic of galactosemia occur: an increase in the size of thymus, spleen and liver. It is established that in 3.5-5 month old rats of the galactose-susceptible substrain the galactoso-1 phosphaturidyltransferase activity in blood hemolysates is 15 times lower than in rats of galactose-resistant substrain, and in liver the activity of this enzyme is 1.4 times lower. The activity of liver galactokinase and uridyldiphosphogalactose-4-epimerase is slightly higher in rats of galactose-susceptible substrain than in galactose-resistant 1.