In Venezuela, four sheep fitted with rumen cannulas were fed the toxin-containing legumes Leucaena leucocephala (sun dried leaves and stems) and Canavalia ensiformis (grains). L. leucocephala did not have any apparent toxic effects on the animals, in spite of the presence of the toxic compounds 3,4 DHP and 2,3 DHP in the rumen. There were no significant effects on the rumen pH, concentrations of VFAs, microbial counts, rate of rice straw degradation or in the rumen outflow of liquids and solids. However, there was an increase in the concentration of ammonia in the rumen, and in the number of Gram negative rods isolated from the rumen of sheep when L. leucocephala was added to the diet. When C. ensiformis was supplemented the presence of canavanine in the rumen was confirmed. Sheep did not show signs of toxicity. There was no effect on rumen pH, microbial counts, degradation of rice straw or rates of outflow of liquids and solids from the rumen. There was a decrease in the rumen concentrations of valerate and ammonia and an increase in the numbers of Gram negative bacteria isolated from the rumen of sheep as the dietary inclusion of Canavalia increased. Several pure cultures capable of mimosine, 2,4 DHP and (in fewer cases) 3,4 DHP wre isolated. One of them, a Gram variable spore-forming rod, presumably a new species of Clostridium, was characterised. The presence of degraders of mimosine and DHP in the rumen presumably accounted for the lack of toxicity of L. leucocephala in Venezuela. The effect of canavanine on the growth and products of fermentation of pure cultures was independent of the Gram staining reaction and the results of experiments using media containing either peptides and amino acids or free amino acids alone suggest that canavanine may have an effect on peptide hydrolysis.