Dynorphin-(1-13) (Dyn-(1-13] and its analogs substituted by single introduction of Ala in positions 1-11 were synthesized by the solid-phase method and purified by high pressure liquid chromatography. Relative potencies of the synthetic compounds were determined by their ability to inhibit electrically-evoked contractions of the guinea pig ileum (GPI) and of the mouse vas deferens (MVD) and to compete with [3H]-etorphine for opiate receptors in rat brain homogenates. Introduction of Ala in positions 1 and 4 of Dyn-(1-13) provoked most important decreases in the activity of the molecule in the three assays (relative potency of 0.2% or less). Substitution of Ala in positions 2 or 5, but not 3, also severely decreased the potency of the peptide in the smooth muscle preparations (0.6-5.0% activity). However, the opiate receptor binding assay was less sensitive to the replacement of residue in position 2 (20% activity) than that in positions 3 or 5 (12% and 6% relative potencies, respectively). In the GPI assay and the opiate binding test, the other substitutions which greatly lowered the potency of the molecule were seen in positions 6, 7, 9 and 11, four basic residues. Among these, Arg6 and Arg7 were demonstrated to be the most important in the three biological tests. Finally, the replacement of Ile8 by Ala increased the relative potency of Dyn-(1-13) up to 191% and 900% in the MVD and the opiate binding tests, respectively.