Sex steroids modify sexual behavior and autonomic function. The gradual decline in circulating levels is correlated with several diseases in humans and animals. However, little is known about age-related changes that occur in the availability of these steroids. In the current studies, we characterized age-related changes in (1) circulating levels of estradiol (females) or testosterone (males), (2) reproductive function (estrous cyclicity in females; erectile reflexes in males), and (3) blood pressure in a longitudinal study. In a separate study, we characterized the estrous cyclicity of sex steroids in female, and diurnal periodicity in male, Long-Evans rats. Young females exhibit regular estrous cycles, transition to irregular cycles at about 10 mo of age, then to cycles characterized by extended periods of estrous, and to persistent estrous. Despite the loss of cyclicity, circulating 17β-estradiol in middle-aged females was maintained at levels similar to those in young females during diestrous. Males display an age-related decline in testosterone, circulating levels decrease by about 25% during the period from 8 to 16 mo of age. Also, during any 24 h period testosterone levels in young males vary from a peak of about 3.5 ng/mL (late light period) to a trough of 0.7 ng/mL (early dark period). In middle-aged males the rhythm amplitude is greatly blunted (1.4 to 0.7 ng/mL). Males exhibit age-related decrements in erectile reflexes. In females and males systolic blood pressure is relatively stable until 8 mo of age, but significantly increases during the next 5 mo of age. In males, the increase in arterial pressure is gradual from about 8 mo of age. Young females have lower blood pressures than age-matched males, but by 14 mo of age this sex-related advantage is lost. Thus, by middle age, male and female rats are exposed to less gonadal hormone/altered patterns of availability, exhibit decrements in reproductive function, and display an increase in systolic blood pressure. © 2004 by Humana Press Inc. All rights of any nature whatsoever reserved.