The effects of some aliphatic alcohols on the activity of several brush border enzymes have been investigated. At the concentrations used we found strong inhibition of(Na + + K +)-APTase, and Mg 2+-APTase while alkaline phosphatase and arylamidase activity was little influenced. The half-maximal ATPase inhibitory concentrations of alcohols were closely correlated with the length of the carbon chain and the interaction with the enzyme appeared to be hydrophobic. The change in free energy due to addition of one CH 2 group is estimated at −620 cal for (Na + + K +-ATPase and −730 cal for Mg 2+-ATPase. The relationship between intestinal pharmacological activity of the alcohols and active transport is discussed.