Changes in the concentrations of glucosinolates from rapeseed meal and some glucosinolate degradation products during incubation in vitro with myrosinase (EC 3.2.3.1), with pepsin (EC 3.4.23.1)-HCl, 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 which 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 to slow the degradation of glucosinolates in the presence of caecal contents.