Researchers developed a method to measure corticosterone levels in rat and mouse brains, with a high recovery rate. They found that brain corticosterone levels change quickly after stress, similar to plasma levels. Different brain regions and subcellular fractions were analyzed for corticosterone distribution. Injecting ACTH in mice increased brain corticosterone levels even when plasma levels were unchanged. This method can help understand the relationship between plasma glucocorticoids and ACTH secretion.