Genetic factors associated with lactose malabsorption were studied in slc-Wistar rats. A decline in the maximum level of blood glucose attained in the lactose tolerance test (LTT) using 1g per kg body weight administered as 20% lactose solution appeared at 5 to 6 weeks of age. The rats were classified into three groups (L, M and H) according to the maximum level of blood glucose upon LTT at 11 weeks of age. Lactase activity and the degree of the decline in the maximum blood glucose level differed among these groups. The heritabilities of the maximum blood glucose level upon LTT and lactase activity were estimated to be 0.4-0.6 and 0.7-0.8, respectively. The foundation population used for the selection experiment was designated as generation 0, and consisted of 124 litters obtained from random mating of slc-Wistar rats. Populations L and H were selected for low and high lactose absorption, respectively based on the maximum blood glucose level upon LTT at 11 weeks of age and on the lactase activity of parents. Population H showed an increase in the maximum blood glucose level and lactase activity, while population L showed a decrease in comparison with the unselected population M. Clinical symptoms such as diarrhea and loose bowels were observed in population L during LTT performed with a lactose dose of more than 1.5 g/kg B.W. These results indicate that lactose intolerance has a high degree of heritability.