Neuronlike, catecholaminergic cells expressing tyrosine-hydroxylase (TH) have recently been found in the testis of a nonhuman primate species, the rhesus monkey. We examined whether neuronlike cells are present in the human testis. To this end, we first determined if the genes for TH and for a voltage-activated sodium channel (NaCh), a prerequisite for neuronal excitability, are expressed in normal adult testes. Using an RT-PCR approach, cDNA clones, identical to the sequences of human TH and to the alpha subunit of a NaCh type, were isolated. Immunohistochemical methods localized the corresponding proteins in testicular biopsies from adult men (age range, 28-44 years) without testicular pathologies and from infertile patients with either Sertoli cell only (SCO) syndrome or severe hypospermatogenesis and germ cell arrest (GA). TH and NaCh antibodies, as well as antibodies recognizing dopamine-transporter protein, identified immunoreactive cells of mainly bipolar or occasionally multipolar, elongated phenotype in most, but not all, biopsies of each group (12 out of 23). The results were corroborated by identification of TH gene expression by RT-PCR approaches in biopsies. Immunoreactive cell bodies, as well as nerve fibers, were more readily detected in SCO and GA biopsies. This was quantified after immunohistochemically visualizing all testicular neuronal elements, cell bodies, and fibers, with a neurofilament 200 (NF-200) monoclonal antibody in one set of randomly selected sections from all biopsies. We found significantly increased NF-200-immunoreactive cell bodies and fibers in SCO-syndrome and GA biopsies. These results show the existence of an as yet unknown testicular catecholaminergic neuronlike cell type in the human testis. This cell type may complement and act in concert with the well-known testicular sympathetic innervation. The increase of both "intrinsic" (neuronal cells) and "extrinsic" (nerve fibers) neuronal elements in pathological testicular biopsies suggests that the two parts of the human testicular nervous system may be involved in pathogenesis and/or maintenance of GA and SCO syndromes.