Leucokinin is a member of the myokinin peptide family. These myotropic peptides are widely distributed in arthropods. A specific antiserum immunoreactive to the neuroactive octapeptide leucokinin I (LKI) has been used to map neurones within the central nervous system of the central American wandering spider Cupiennius salei. The antiserum labels nine pairs of cell bodies and their axons. The somata are grouped near the joint of the optic lobes in the dorsal part of the supraoesophageal ganglion. The axons descend in a bundle, pass the oesophagus and divide to innervate the suboesophageal ganglion at three levels. In the dorsal-most layer, five small parallel fibres travel within the medio-central tract towards the opisthosomal neuromeres and give off small varicose projections into all leg neuromeres. In the middle layer, two neurones in the sensory-longitudinal tract 3 project into each leg neuromere. In the ventral layer, the two largest fibres (about 8 microm) run in the medio-ventral tract, each giving off a branch into all leg neuromeres. These first order arborizations have small second order arborizations in the ventral part of the leg neuromere and ascend to the five dorsal fibres in the medio-ventral tract where they form varicosities. The second order arborizations form extensive varicosities in the ventral part of the leg neuromere. These neurones may play a role in the intercellular communication between the sensory input and the motor output system, because they are represented in the dorsal sensory and the ventral motor neuropile. The ventral fibres resemble intersegmental interneurones and may function as modulators for leg motor neurones.