The formation of [3H]oestradiol macromolecule complexes was studied in vivo and in vitro in the kidney, lung and liver of intact foetal guinea pig. Specific binding of [3H]oestradiol was demonstrated in the cytosol and nuclear fractions of foetal kidney. Intensive binding was found in the cytosol of foetal lung but most of the bound radioactive material (70 %) was [3H]oestrone. Some binding was found in the cytosol of foetal liver but not in the nucleus of this tissue. In the in vivo experiments, the binding of radioactive material to plasma proteins was studied: 22 % of the plasma radioactivity was bound of which 67 % was identified as oestrone sulphate. Oestradiol sulphate represented 7% and unconjugated oestradiol only 0.6 % (0.1 % of the total plasma radioactivity). On the other hand, 2-3 % of the foetal plasma radioactivity was found as unbound [3H]oestradiol. In the kidney, the formation of [3H]oestradiol complexes in the cytosol fraction does not depend on temperature while nuclear [3H]oestradiol complexes increase with increasing temperature. Maximal formation of [3H]oestradiol complexes in the cytosol fraction and the 0.1 M TRIS and 0.3 M NaCl nuclear extracts was reached after 15 min but binding in the 1 M NaCl nuclear extract continued to increase up to 30 min. After incubation of purified nuclei of foetal kidney with 1.1 x 10(-7) M [3H]oestradiol, specific binding was found in the different nucelar fractions. Specific binding was also detected in isolated nuclei previously extracted by 0.1 M TRIS and 0.3 M NaCl before being incubated with 1.1 x 10(-7) M [3H]oestradiol. The Kd of binding of [3H]oestradiol in the renal cytosol fraction is 2.5 x 10(-10) M with n = 4.5 x 10(-14) moles/mg protein. Incubation of isolated 1 M NaCl nuclear extract from foetal kidney with [3H]oestradiol gives a Kd of 3.3 x 10(-10) M with n = 2.5 x 10(-14) moles/mg protein. It is concluded that the nuclear complexes in the foetal kidney could be formed either through an intermediate cytosol complex or that the "two step" mechanism could take place in the nucleus. Furthermore, direct binding of [3H]oestradiol with high affinity was observed in the 1 M NaCl nuclear extract.