The inhibition of reflex urinary bladder contractions, recorded isometrically in the urethane-anesthesized rat, was used as an index of central opioid activity. Inhibition of bladder activity has been shown to be mediated both spinally and supraspinally by mu and delta opioid receptors. Using this model a number of neuropeptide fragments of the proenkephalin A molecule (peptide F, peptide E, BAM 22P, BAM 12P, Met5-enkephalin-Arg6,Phe7,Leu8, Met5-enkephalin-Arg6,Phe7, Met-enkephalin, Leu-enkephalin) were tested for in vivo activity. Each of the fragments inhibited reflex bladder contractions when administered by bolus microinjection into a lateral ventricle (i.c.v.) or intrathecally into the spinal subarachnoid space (between L3 and L4 vertebra). The larger molecular weight peptides produced more prolonged inhibition of bladder activity than the smaller molecular weight ones, with the rank order of activity being similar at spinal and supraspinal sites: peptide F greater than or equal to peptide E = BAM 22P greater than BAM 12P greater than Met5-enkephalin-Arg6,Phe7,Leu8 = Met5-enkephalin-Arg6,Phe7 greater than or equal to Met-enkephalin = Leu-enkephalin. The relative receptor selectivity of each fragment was further determined using the opioid antagonists naloxone (mu receptors) and ICI 174,864 (N, N-diallyl-Tyr-Aib-Aib-Phe-Leu-OH: Aib = alpha-aminoisobutyric acid) (delta receptors). The activity (Ke) of each antagonist against equieffective doses of the highly selective opioid receptor ligands [D-Ala2,Me-Phe4,Gly(ol)5]enkephalin (mu receptors) and [D-Pen2,D-Pen5]enkephalin (delta receptors) was compared with that against the proenkephalin A fragments.(ABSTRACT TRUNCATED AT 250 WORDS)