A rapid and sensitive radioenzymatic assay for measuring catechol estrogens in tissue has been developed. This assay is based on converting the relatively labile catechol estrogens to stable O-methylated derivatives by the enzyme catechol-O-methyl-transferase. Using a radioactive methyl donor of high specific activity (3H-S-adenosylmethionine) and solvent extraction with non-polar solvents a sensitivity of 25 pg can be achieved. The specificity of this assay was confirmed by thin layer chromatography and mass spectral analysis of the reaction products. Catechol estrogens in rat liver are significantly decreased after ovariectomy and markedly increased by treatment with estrogen.