The possibility that opioid peptides and noradrenaline co-exist not only in the desheathed bundle of bovine splenic nerve which contains approximately 98% sympathetic C-fibers, but also in the population of large dense-cored noradrenergic vesicles from these fibers, has been investigated. The primary fraction of large dense-cored vesicles which can be prepared at about 85% purity has been further subjected to density gradient and fractional centrifugation procedures, including D2O-loading and unloading on modified second gradients, in an attempt to separate any minor population of particles which potentially could contain opioid peptides and contaminate the large dense-cored vesicle fraction. Measurement of opioid peptides, noradrenaline, dopamine and dopamine beta-hydroxylase activity supports the conclusion that opioid peptides are stored in the primary population of large dense-cord vesicles per se, rather than in a minor population of contaminating particles from cells other than sympathetic C-fibers. This conclusion has implications for exocytotic release and the physiological role of the opioid peptides intra- and extra-neuronally. Nerve vesicle opioid peptides have a size less than 5000 daltons, in contrast to the high proportion of large peptides containing enkephalin sequences in the bovine adrenal medulla.