The uteri obtained from the estrogen-treated rabbits were incubated with [35S] sulfate for 6 hr. The medium and the tissues were then separated from the incubation mixture and digested with pronase. Each digest was fractionated by DEAE-Sephadex A-25 (Cl- form) column chromatography. The radioactivity in the subfractions (M-0.9 M Fr and EI-0.9 M Fr) eluted with 0.9 M NaCl was the highest among six subfractions in both cases. The compositions of sulfated glycosaminoglycans in M-0.9 M Fr and EI-0.9 M Fr were examined by electrophoresis on cellulose acetate membrane and by gel filtration through Sephadex G-50, in combination with nitrous acid treatment and chondroitinase AC II digestion. The electrophoretograms indicated that sulfated glycosaminoglycans in EI-0.9 M Fr were chondroitin sulfates A and (or) C, heparan sulfate and dermatan sulfate, while those in M-0.9 M Fr were mostly chondroitin sulfates A and (or) C. Although heparan sulfate in M-0.9 M Fr was not detected on the electrophoretogram by staining with Alcian blue, it was considerably labelled with [35S]sulfate. This finding suggested that the specific radioactivity of this heparan sulfate was very high. The present data indicate that the estrogen-treated rabbit uterus actively synthesized sulfated glycosaminoglycans.