Changes in the concentrations of glucosinolates from rapeseed meal and some glucosinolate degradation products during incubation in vitro with myrosinase (EC 220.127.116.11), with pepsin (EC 18.104.22.168)–HCI, and with contents of porcine small intestine and caecum were studied. When rapeseed meal was incubated with myrosinase, 5-vinyl oxazolidinethione (OZT) and butenyl and pentenyl isothiocyanates were produced; OZT concentration rose to a plateau after about 2 h. However, when incubated with caecal contents only OZT could be detected; its concentration peaked after about 4–5 h then declined. Under in vitro conditions whirh attempted to simulate peptic and small intestinal digestion no OZT could be detected; the individual glucosinolates differed in susceptibility to peptic conditions, losses ranging from 3 to 23%. Under the small intestinal conditions the losses of individual glucosinolates ranged from about 7 to 28%. Addition of CuSO4, ascorbic acid, tylosin or a probiotic had little effect on the outcome of peptic or small intestinal incubations but tylosin appeared tc slow the degradation of glucosinolates in the presence of caecal contents.