The major pelvic ganglion in the adult rat differs in neuron number in the two sexes. The female pelvic ganglion has 5892 +/- 797 (mean +/- S.D.) neurons while the male contains 14,654 +/- 936. This sexual dimorphism in neuronal numbers in the autonomic nervous system is of interest because it suggests that endogenous hormone levels during development may have an influence on numbers of neurons in the adult pelvic ganglion.