By the use of light microscopic (LM) immunohistochemistry the distribution of tachykinin (TK)-, calcitonin gene-related peptide (CGRP)- and neuropeptide Y (NPY)-like immunoreactivity in nerves supplying the mammalian (rat, mouse, guinea-pig, cat) thymus gland has been determined. There were no interspecies variations. Fibres staining for TK and CGRP completely overlapped indicating coexistence. They were present in the capsule, in interlobular septa and in the corticomedullary boundary and occurred in perivascular and paravascular plexus supplying arteries, veins and the microvasculature. Some TK/CGRP-immunoreactive (ir) fibres travelled between lymphoid cells and close contacts with mast cells were frequent. NPY-ir fibres were different from those staining for TK/CGRP and predominated in the perivascular plexus of arterial blood vessels. Only very rarely they coursed in the lymphoid parenchyma. Intimate contacts of NPY-ir fibres with mast cells were less frequent than those of TK/CGRP-ir fibres. We conclude that the NPY innervation is mainly sympathetic noradrenergic while thymic nerves coding for TK and CGRP are most likely of sensory origin. These pathways may play a differential neuroimmunomodulatory role in the thymus, possibly via interaction with mast cells.