The numbers of neurons in the rat superior cervical sympathetic ganglion (SCG) differ in males and females, with the males having 30% more SCG neurons than females at 60 days of age. This sex difference arises during the early postnatal period, when testosterone administration increases the numbers of neurons and alters the nerve growth factor (NGF) content of the rat SCG. In contrast, there is no gender difference in number of neurons in the L1 dorsal root ganglion. In both males and females, the amount of NGF bound per ganglion increased between postnatal days 5 and 15 (P5 and P15) in both dorsal root ganglia (DRGs) and the SCG. There is also a gender difference in NGF binding: SCGs and DRGs of female rats at both P5 and P15 bind more NGF per ganglion than do those of males. This effect was more marked in DRGs than in the SCG. Treatment of neonatal females with testosterone reduced NGF binding in both SCGs and DRGs to levels comparable to males at P5, and in DRGs at P15. In contrast, treatment of males with testosterone from birth resulted in a 2-3 fold increase of NGF binding in both SCGs and DRGs as compared to controls at P15. At P15, testosterone treatment of females increased NGF binding in the SCG. Males and females had opposing responses to neonatal exposure to estradiol. Treatment with estradiol from birth increased NGF binding in SCGs and DRGs of females, but had no effect on NGF binding of SCGs, and reduced NGF binding in DRGs of males.(ABSTRACT TRUNCATED AT 250 WORDS)