Glucosinolates are a group of plant thioglucosides, found in a range of plant genera, notably the Brassicae. Glucosinolates assume agricultural importance because of their presence in rapeseed meal and forage brassica crops whose nutritional value they may limit. Glucosinolates are hydrolysed under the action of plant or microbial myrosinase to release a range of toxic metabolites (see Duncan, 1991). For example, sinigrin, a commonly found glucosinolate, breaks down to allyl isothiocyanate (AITC) and allyl cyanide as well as a number of lesser components. The course of hydrolysis is influenced by characteristics of the hydrolysis environment such as pH and the presence of metallic ions and enzymic co-factors. Because of the varied nature of the effects of different glucosinolate metabolites, the course of hydrolysis is a key influence on the ultimate toxicity of glucosinolates. Little is known, however, of the identity of metabolites arising in the digestive tract of livestock under different dietary circumstances. This is due, in part, to the technical difficulties associated with measuring concentrations of the compounds in the digestive tract. An alternative approach to the study of glucosinolate hydrolysis in vivo is to measure concentrations of ultimate excretory products and relate this to events earlier in the catabolic pathway. For example, isothiocyanates are excreted as their mercapturic acid derivatives in rats and thus provide potentially useful markers for estimating rates of release of isothiocyanates from sinigrin in vivo (Mennicke et al. 1983). In the experiment reported here, this approach was validated in rats and used to measure rates of release of AITC from sinigrin in animals consuming diets with or without added myrosinase.