Various agents that have been reported to reduce the enzymatic degradation of the enkephalins have been tested for their ability to potentiate the activity of [Met5]enkephalin in three in vitro assay tissues. The greatest effect was obtained with the combination of bestatin (10 microM or 30 microM), captopril (10 microM), thiorphan (0.3 microM) and L-Leucyl-L-leucine (2 mM) which increased the potency of [Met5]enkephalin 18-fold in the guinea-pig myenteric plexus, 13-fold in the mouse vas deferens and 200-fold in the rat vas deferens. The increased potency is attributed to inhibition of the peptidases since the mixture of inhibitors did not change the activity of either normorphine or the metabolically stable synthetic opioid peptides. The potencies of the hexa-, hepta- and octapeptide C-terminus extensions of [Met5]enkephalin and [Leu5]enkephalin were increased by the peptidase inhibitors in all three preparations; the greatest effects were found in the rat vas deferens. No significant changes in the potencies of fragments of beta-endorphin longer than beta-endorphin-(1-19) were obtained. It may now be possible to inhibit enzymatic degradation of opioid peptides sufficiently to measure their release from neurones activated by electrical field stimulation.