The toxicity of mirex, an organochlorine insecticide, is increased significantly in adrenalectomized rats. To determine if this might be due to alterations in the tissue distribution of mirex, a disposition study was conducted in intact, adrenalectomized, and adrenalectomized corticosterone-supplemented rats. The peak plasma levels of [14C]mirex were shifted from 2 hr in intact and adrenalectomized corticosterone-supplemented rats to 4 hr in the adrenalectomized rats. The initial uptake and disposition of [14C]mirex in the brain of the intact, adrenalectomized, and adrenalectomized corticosterone-supplemented rats was essentially the same from 0.5 to 6 hr after dosing. By 48 hr after dosing, the [14C]mirex concentration in the brain of intact and adrenalectomized rats was significantly higher than that in adrenalectomized corticosterone-supplemented rats. Brain to plasma ratios reflected this difference. Corticosterone supplements to adrenalectomized rats decreased the absorption of [14C]mirex in the brain. The uptake of [14C]mirex during the initial 6 hr after dosing in all other tissues studied (liver, kidney, testes, and omental fat) was essentially the same for the three treatment groups. By 48 hr postdose, [14C]mirex concentration per liver was significantly higher in intact than in adrenalectomized and adrenalectomized corticosterone-supplemented rats. The difference in [14C]mirex disposition between the three treatment groups was not due to alterations in metabolism or elimination.