A growing area of research in pharmacology and toxicology is concerned with the role of adrenal glucocorticosteroids (predominantly corticosterone in rats and mice, and cortisol in humans) in modulating toxicological responses. These steroids are secreted from the adrenal cortex, and in laboratory rodents secretion occurs particularly in response to stressful environmental change or noxious challenge, which can include a toxic insult. Glucocorticoids have profound biochemical effects in diverse tissues (e.g., inhibition of DNA and RNA synthesis; effects on carbohydrate metabolism, protein catabolism, and immunosuppression, and anti-inflammatory effects). Given this range of pharmacological actions of glucocorticoids, effects on the response and tolerance to a toxic insult can be hypothesised. Indeed, it is now becoming clear that the toxicological response to a toxin can be modulated by pretreatment with, or coadministration of, natural glucocorticosteroids or their synthetic analogues. As achieving the maximum tolerated dose (MTD) in an experimental animal, whether via a natural toxin or a synthetic chemical, can be stressful, the implications of these findings are far reaching. This review is intended to illustrate the principle that the toxicological responses to natural toxins (e.g., kainic acid, aflatoxin B1, trichothecenes) can be modulated by corticosterone and other natural and synthetic glucocorticosteroids. Particular emphasis is given to neurotoxicity and hepatotoxicity in laboratory rodent models, but nephrotoxicity and cardiotoxicity, of which there are fewer studies, are also covered. Glucocorticoids can both enhance toxicity or protect against toxicity, and the direction of the effect depends on the target organ, the particular steroid, the properties of the toxin, and the temporal relationship of the coadministration regime.