Sexual dimorphism of the genitofemoral nerve spinal nucleus has been demonstrated in normal rodents. Calcitonin gene-related peptide is a neurotransmitter, present in the genitofemoral nerve, that has been implicated in the regulation of gubernacular migration and inguinoscrotal testicular descent. A combination of retrograde fluorescent labelling of the genitofemoral nerve and immunohistochemistry for calcitonin gene-related peptide was used in 1-3-day-old mutant TS rats with 85% incidence of congenital cryptorchidism and an absence of the normal sexual dimorphism of the genitofemoral nerve spinal nucleus was demonstrated. There was no significant difference between male and female nuclei with respect to fluorescent-labelled neurones as well as those immunoreactive for calcitonin gene-related peptide, in contrast to an obvious sexual dimorphism present in normal control animals. This lack of normal sexual dimorphism of the genitofemoral nerve nucleus is likely to be important in the pathogenesis of cryptorchidism in this animal model.