Total and specific lactase activities in the small intestine of Chester White and Hampshire pigs were measured at 1, 8, 15, 22, 29, and 43 days of age. The small intestine was divided into 10 segments of equal length, the proximal 10 cm of each segment was scraped, and the scrapings were homogenized for use in the lactase determinations. Significant breed, age, and segment differences were observed for both specific and total activities. In both breeds, the total lactase activity at 1 day of age was lower than that at any other age. After reaching maximal levels at 15 days of age, the total activity declined with the loss of activity occurring primarily in the ileum. At 1 and 8 days of age, the total lactase activities for the two breeds were similar, but the Chester White pigs had higher activities at all other ages. The pattern of changes in specific activity with age was similar for both breeds. The mean specific activity was highest at 1 and 8 days of age and then fell progressively to minimal levels at 43 days of age. Chester Whites had higher specific activities than Hampshires during the first 4 weeks of life, but at 6 weeks of age there was little difference between the breeds. The peak lactase activity, expressed as total or specific activity, occurred in the proximal one-third of the small intestine of both breeds, and the distal one-third of the gut had relatively low activities as the animals matured.