Our objective was to determine the short-term effects of feeding colostrum and mature milk on the enzyme activity and relative isoform abundance of lactase in neonatal pigs. We studied newborn pigs that were fed colostrum, mature milk and water for 6 h (Study 1). We also studied unfed pigs shortly after birth, and newborn pigs fed either colostrum or mature milk for 24 h (Study 2). The specific activity (micromol x min(-1) x g protein(-1)) of lactase in ajejunal mucosal homogenate and a purified membrane fraction was lower in pigs fed colostrum than in unfed newborns or those fed either milk or water. However, after 24 h, total jejunal lactase activity (micromol x mol(-1)), jejunal mass and protein content were higher in the colostrum- and milk-fed pigs than in the unfed newborns. In colostrum-fed pigs, the reduction in lactase specific activity after 6 h was associated with 1) a marked increase in the relative abundance of a 180-kDa protein, which was shown to be one of three pro-lactase isoforms, and 2) a lower relative abundance of the 160-kDa isoform, considered to be the mature form of the enzyme. Our evidence suggests that feeding either colostrum or mature milk increases total jejunal lactase activity. The reduction in both the specific activity and abundance of the mature isoform in conjunction with an increased relative abundance of the 180-kDa pro-lactase isoform suggests that feeding colostrum alters the post-translational processing of intestinal lactase in neonatal pigs.