Binding of [3H]triamcinolone acetonide (TA) to cytosolic receptors and subsequent in vitro activation of glucocorticoid-receptor complexes were studied in whole brain and liver from misty diabetic mice (mdb/mdb) and their control littermates (??/++). Binding was specific for glucocorticoid receptor (GcR) since the specific glucocorticoid, RU26988, was used to compete with [3H]TA for binding. Reduced [3H]TA binding was observed in whole brain and liver in diabetic animals when compared to control animals. Within the brain, binding was significantly (P less than 0.05) decreased in cortex, hippocampus, and hypothalamus. No significant differences in binding were found in the striatum or "midbrain". GcR binding was similar in diabetic and control animals until 2 months of age when overt diabetic symptoms appeared and the GcR binding was lower in diabetic animals. Though GcR from mdb/mdb brain cytosol could be thermally activated, the extent of activation was significantly (P less than 0.05) less than that for controls. These data indicate that GcR in liver and brain cytosol are decreased in mdb/mdb mice and that the GcR available for binding in mdb/mdb brain cytosol appears less capable of undergoing activation and binding to DNA-cellulose than GcR from control brain cytosol. Decreased GcR activation in brain cytosol from mdb/mdb mice was associated with increased dissociation of [3H]TA from the GcR. These results suggest that the decreased negative feedback previously observed in diabetic animals may be due to decreased binding of hormones and a decreased level of activation of hormone bound receptor complexes.