Increased intake of L-tryptophan from lush pastures and its subsuquent microbial degradation in the rumen is associated with pulmonary oedema and emphysema. Tryptophan is degraded in the rumen to the toxic metabolite 3-methylindole in a two-step degradation process. The rates of tryptophan degradation, however, depends on both availability of the substrate and dietary intake. There appears to be a greater tryptophan rate of conversion in ruminants fed on forage diets than those fed concentrate diets. Sudden dietary changes from a sparse feed to a lush, succulent pasture might cause acute bovine pulmonary emphysema, a respiratory disease of cattle. There is no effective treatment for this condition, and prevention by avoiding sudden dietary changes to lush pastures seems to be the only alterrnative.