The 17 beta-estradiol-induced increase of uterine blood flow (UBF) was studied in 7- to 8-month-old oophorectomized rabbits 6-39 days after operation. Baseline blood flows were determined with 141Ce-labeled microspheres and 2-h flows with 85Sr-labeled microspheres. Mean UBF in control groups ranged from 0.11-0.26 mi/min . g. Mean UBF 2 h after 17 beta-estradiol (10 microgram/kg BW) was 1.51 ml/min . g. Utilizing [3H]uridine (100 microCi/kg 15 min before the 2-h flow study), 3H incorporation into uterine RNA was 2-fold higher than background. Estradiol treatment produced a 4-fold increase in 3H incorporation into uterine RNA. Estradiol-treated animals, pretreated with actinomycin D (4 mg/kg), increased UBF to 1.09 ml/min . g 2 h after estradiol but failed to increase uterine RNA synthesis. 3H incorporation into uterine RNA in this group was suppressed to undetectable (background) levels. These findings indicate that a mechanism other than accelerated transcription, whether induced directly or by translocation of a cytosolic receptor, underlies the increased UBF after estradiol administration to oophorectomized rabbits.