[6,7-3H] Estrone (E) and [6,7-3H]estradiol-17 beta (E2) have been synthesized by reduction of 6-dehydroestrone and 6-dehydroestradiol with tritium gas. Tritiated E and E2 were administered by oral gavage to female rats and to male and female hamsters on a dose level of about 300 micrograms/kg (54 mCi/kg). After 8 h, the liver was excised from the rats; liver and kidneys were taken from the hamsters. DNA was purified either directly from an organ homogenate or via chromatin. The radioactivity in the DNA was expressed in the units of the Covalent Binding Index, CBI = (mumol chemical bound per mol DNA-P)/(mmol chemical administered per kg b.w.). Rat liver DNA isolated via chromatin exhibited the very low values of 0.08 and 0.09 for E and E2, respectively. The respective figures in hamster liver were 0.08 and 0.11 in females and 0.21 and 0.18 in the males. DNA isolated from the kidney revealed a detectable radioactivity only in the female, with values of 0.03 and 0.05 for E and E2, respectively. The values for male hamster kidney were less than 0.01 for both hormones. The minute radioactivity detectable in the DNA samples does not represent covalent binding to DNA, however, as indicated by two sets of control experiments. (A) Analysis by HPLC of the nucleosides prepared by enzyme digest of liver DNA isolated directly or via chromatin did not reveal any consistent peak which could have been attributed to a nucleoside-steroid adduct. (B) All DNA radioactivity could be due to protein contaminations, because the specific activity of chromatin protein was determined to be more than 3,000 times higher than of DNA. The high affinity of the hormone to protein was also demonstrated by in vitro incubations, where it could be shown that the specific activity of DNA and protein was essentially proportional to the concentration of radiolabelled hormone in the organ homogenate, regardless of whether the animal was treated or whether the hormone was added in vitro to the homogenate.(ABSTRACT TRUNCATED AT 250 WORDS)