The proximity effects observed in calculating octanol/ water partition coefficients of bisubstituted aliphatic chains are interpreted as a measure of the hydrophobicity potential of the substituents. These effects and the derived potentials decay exponentially from the center of the molecular fragment (substituent). A hydrophobicity potential is defined that is proportional to the hydrophobic fragmental constant and has its maximal value in the center of the fragment. Summing up the fragmental hydrophobic contributions enables us to assign a potential to any point in space around the molecule. Selected colored representations of the potential, such as those shown here for enalapril, add to the available pictures of bioactive molecules and should be useful for design purposes in molecular pharmacology. © 1988.