Cutaneous sympathetic vasoconstrictor nerves are tonically active; changes in their activity regulate skin blood flow and are important in thermoregulation. Studies in humans have shown that female reproductive hormones have substantial influences on thermoregulatory responses (1) and that before the menopause, the vasoconstrictor influences of sympathetic nerve fibres are greater in females than males (2). Sympathetic nerve activity to skeletal muscle vasculature increases with age in both women and men, but is lower in women than men in the 20-29 year age range (3). Whether the same is true of cutaneous sympathetic vasoconstrictor nerves is not known, nor whether any differences between males and females exist pre-puberty. Further, it is not known whether there are differences in sympathetic nerve innervation density that might contribute to sex-related differences in thermoregulation. Thus, we have investigated the effect of sex and maturation on sympathetic nerve innervation density of the CVA of the tail, the main thermoregulatory organ in the rat. Four groups (n=6) of Wistar rats were used: juvenile male and female (4-weeks) and sexually mature (12-weeks) male and female, approximately equivalent to 10 year-old and 30 year-old humans. A length of CVA was removed under anaesthesia (Alfaxan 12 mg.kg^-1.hr^-1 i.v.). The catecholamine-containing sympathetic nerve fibres were visualised using the glyoxylic acid method and quantitative analysis was performed to measure surface density of the nerves on the blood vessel (4). Comparisons were made using Students unpaired t-test, P<0.05 being considered significant. The innervation density of the CVA of female juvenile rats was significantly lower than that of males (0.049±0.003 vs 0.056±0.001 interceptions.µm^-1). However, there was no difference between the innervation densities of the CVA in sexually mature female and male rats (0.045±0.002 vs 0.047±0.003 interceptions.µm^-1). Thus, interestingly, innervation density significantly decreased with maturation in males, but not in females. Thus, the present results indicate that in juvenile (pre-pubescent) rats, the sympathetic innervation density of an artery with major thermoregulatory function is lower in females than males. However, the innervation densities are similar in sexually mature males and females because density decreases with maturity in males, but not in females. Given sympathetic vasoconstrictor influences are greater in sexually mature pre-menopausal women than in age-matched men (2), we suggest that female reproductive hormones preserve or enhance the influence of sympathetic nerve fibres on thermoregulatory vessels. Post-menopausally, sympathetic nerve activity is higher in females than males (3), so raising the question as to how the menopause affects sympathetic innervation density.