Biosynthetic pathways for the formation of neuroactive peptides and the processes for their inactivation include several enzymatic steps. In addition to enzymatic processing and degradation, several neuropeptides have been shown to undergo enzymatic conversion to fragments with retained or modified biological activity. This has most clearly been demonstrated for e.g. opioid peptides, tachykinins, calcitonin gene-related peptide (CGRP) as well as for peptides belonging to the renin-angiotensin system. Sometimes the released fragment shares the activity of the parent compound. However, in many cases the conversion reaction is linked to a change in the receptor activation profile, i.e. the generated fragment acts on and stimulates a receptor not recognized by the parent peptide. This review will describe the characteristics of certain neuropeptide fragments having the ability to modify the biological action of the peptide from which they are derived. Focus will be directed to the tachykinins, the opioid peptides, angiotensins as well as to CGRP, bradykinin and nociceptin. The kappa opioid receptor selective opioid peptide, dynorphin, recognized for its ability to produce dysphoria, is converted to the delta opioid receptor agonist Leu-enkephalin, with euphoric properties. The tachykinins, typified by substance P (SP), is converted to the bioactive fragment SP(1-7), a heptapeptide mimicking some but opposing other effects of the parent peptide. The bioactive angiotensin II, known to bind to and stimulate the AT-1 and AT-2 receptors, is converted to angiotensin IV (i.e. angiotensin 3-8) with preference for the AT-4 sites or to angiotensin (1-7), not recognized by any of these receptors. Both angiotensin IV and angiotensin (1-7) are biologically active. For example angiotensin (1-7) retains some of the actions ascribed for angiotensin II but is shown to counteract others. Thus, it is obvious that the activity of many neuroactive peptides is modulated by bioactive fragments, which are formed by the action of a variety of peptidases. This phenomenon appears to represent an important regulatory mechanism that modulates many neuropeptide systems but is generally not acknowledged.