To better understand the effect of dietary fat on intestinal brush border (BB) membranes in the young animal, we compared the effect of dietary medium-chain triglycerides (MCT) with that of monounsaturated and saturated long-chain triglycerides (LCT) on jejunal brush border membrane lipid composition and hydrolase activity in newly weaned rats. Twenty-day-old rat pups were divided into three groups and were weaned to diets containing 14% MCT + 6% soybean oil, 18% olive oil + 2% soybean oil, or 14% tallow + 6% soybean oil, and fed for 40 h or for 33 d. The diets were isonitrogenous and contained similar amounts of cholesterol and polyunsaturated fatty acids. Within 40 h, the fatty acid compositions of the brush border membranes were significantly different among treatment groups. These differences were maintained in rats fed for 33 d. No medium-chain fatty acids, but significantly greater amounts of polyunsaturated fatty acids, especially 18:2(n-6) and 20:4(n-6), were found in the brush border membranes of rats fed the medium-chain triglyceride diet. The cholesterol and phospholipid concentrations in the membranes were highest in rats fed the medium-chain triglyceride diet for 33 d. Rats fed that diet for 40 h had generally higher leucine aminopeptidase, sucrase and maltase activities compared with rats fed the olive oil or tallow diets. However, after 33 d of feeding, the differences between dietary treatment groups disappeared. This study demonstrates that, in the newly weaned rat pup, dietary medium-chain triglycerides and long-chain triglycerides rapidly affect the fatty acid composition of the brush border membrane. However, the changes in the hydrolase activities associated with the changes in the lipid composition of the membranes are transient.