Because dried whey contains approximately 70% lactose, it could be harmful if incorporated into the diet of animals with low tolerance for lactose. Three experiments were conducted to determine the effects of various levels of dried whey in the diet of growing pigs. In the first two experiments diets containing up to 40% dried whey were fed from weaning to approximately 5 months of age. With respect to rate of gain or feed efficiency, there were no significant differences among dietary treatment groups. In a third experiment pigs that had consumed a diet containing no lactose from 6 to 12 weeks of age performed normally when fed a diet containing 40% dried whey from 12 to 21 weeks of age. The results of the three experiments suggest that the growing pig can tolerate up to 30% lactose in the diet without any symptoms of lactose intolerance, and that continuous exposure to lactose in the diet is not necessary to maintain tolerance to this level of lactose.