Several, though not all, estrogen-dependent phenomena show reduced responsiveness to estradiol (E2) during aging. One factor contributing to this reduced sensitivity could be an increase in the dissociation rate of the estrogen receptor-hormone complex. We therefore studied the effect of aging on the dissociation rate of 3HE2 from the uterine cytosolic estrogen receptor (ER) of C57BL/6 mice that had been ovariectomized 48 h earlier. Measurements were made at 28 degrees C at two concentrations of cytosol. In dilute cytosol ([ER] = 0.01 nM) the dissociation of ER-3HE2 displayed a single phase, first order profile which did not differ among young (4-6 month), middle-aged (15-18 month) and old (23-30 month) mice. In more concentrated cytosol ([ER] = 2 or 6 nM) the dissociation of ER-3HE2 displayed a biphasic first order profile that consisted of an initial rapidly dissociating phase followed by a more slowly dissociating phase. There was no effect of age on the dissociation rate constant (K) of either the rapid (K-1) or slow (K-2) phase. Shortening the time allotted for the concentrated cytosol to equilibrate with 3HE2 before measuring the dissociation rate reduced the fraction of the receptor hormone complex that dissociated in the slow phase, but, once again, the dissociation profiles did not differ between age groups. These results indicate that uterine ER dissociation kinetics remain unaltered in middle-aged and old mice and are therefore unlikely to play a role in the attenuated responsiveness to estrogen during aging.