The glomerular ultrafiltration rate varies as a function of age and sex. To further elucidate the basis for the sexual difference, an androgen [Deca-Durabolin (DECA)] was administered to female ovariectomized rats, and glomerular hemodynamics were evaluated by renal micropuncture after 6 and 16 weeks of therapy. Results were compared to those in control ovariectomized female rats injected with vehicle. Therapy did not produce significant differences in body weight, but kidney size was modestly increased in DECA-treated rats at 6 weeks (0.68 +/- 0.03 vs. 0.86 +/- 0.03 g wet weight; P less than 0.05); at 16 weeks major differences in renal size were documented (0.69 +/- 0.03 vs. 1.18 +/- 0.05 g wet weight; P less than 0.01). The increase in size was primarily due to tubular hypertrophy, with more modest increases in glomerular size. After 6 weeks of therapy, the single nephron glomerular filtration rate (SNGFR) was increased in DECA-treated ovariectomized rats (24.8 +/- 1.0 vs. 32.9 +/- 1.1 nl/min; P less than 0.01). Whole kidney glomerular filtration rate also rose in proportion to increases in kidney size. The greater SNGFR was attributed to higher rates of nephron plasma flow and a numerical increase in the glomerular ultrafiltration coefficient. However, after 16 weeks of androgen therapy, in spite of marked renal hypertrophy, SNGFR did not further rise in proportion to renal size, and the rate of nephron plasma flow and the glomerular ultrafiltration coefficient actually fell relative to those in control untreated rats. Light microscopic evaluation of renal tissue revealed no abnormalities in DECA-treated rats. Thus, 6-week androgen therapy to ovariectomized female rats increased both glomerular ultrafiltration rates and renal size. However, with prolonged administration a glomerular dysfunction may have ensued whereby glomerular ultrafiltration was dissociated from increases in renal size.