The leucokinins (LKs) are a group of eight related peptides isolated from the cockroach, Leucophaea maderae. 2. Antisera raised against LK-V, which were specific for the conserved mid to C-terminal region of the LKs, were used to immunostain the parasitic nematode Ascaris suum. 3. LK-IR was observed in neurons in the anterior nerve ring, retrovesicular ganglion, and ventral and dorsal nerve cords of the parasite. Immunostaining was specific in that it was abolished by preabsorption of the antiserum with different leucokinins. Some of the LK-immunoreactive neurons were identified on the basis of their morphological similarity with identified neurons in the free-living nematode Caenorhabditis elegans. 4. Immunoreactivity towards a number of other peptides, notably neuropeptide F, FMRF-amide, KGQELE and KELTAE, has previously been demonstrated in all LK-immunoreactive neurons, thus confirming the multiple-peptidergic nature of certain nematode nerves. 5. LK-IR was demonstrated and quantified in a number of tissues using RIA. Highest amounts were found in extracts of gut; LK-IR was also demonstrated in extracts of body wall, heads and tails, testes, ovaries and pseudocoelomic fluid. 6. The distribution of tissue LK-IR did not correlate with the amount of neuronal tissue in the samples. 7. Dilution experiments suggested that the LK-IR in the parasite is heterogeneous and that the peptide(s) in some tissues may not be analogous to the insect LKs. 8. The LK-IR in the parasite remains to be characterized; however, the results suggest that peptides related to the leucokinins may have a far wider phylogenetic distribution than has hitherto been thought.
