The potencies of a series of enkephalin derivatives with general structure H . Tyr-D-Ala-Gly-X-Y . NH2 (X and Y are variable amino acid residues), to depress the contractions of electrically stimulated guinea pig ileum and mouse vas deferens preparations, were analyzed. The doses for half-maximal inhibition could be expressed as linear functions of three structural parameters--electronic, hydrophobic, and steric--which characterized the side chains of X and Y. A general methodology was devised for the quantitative estimation of these substituent parameters for amino acid side chains. The excellent statistics obtained for the equation of regression is an indication that no other parameters need to be considered to account for the opiate activity in this series. The relative importance of these factors and their intercorrelation were established, and the predictive value of the model was tested.