The effect of Brachiaria decumbens (signal grass) on drug-metabolizing enzymes was studied in sheep. After 14 d of grazing a pure signal grass pasture, significant declines were observed in hepatic aminopyrine N-demethylase and aniline 4-hydroxylase (phase I biotransformation) and in conjugative enzymes UDP-glucuronyltransferase and glutathione S-transferase. Kidney enzymes were significantly decreased except for UDP-glucuronyltransferase. Enzyme activities were also compared for normal sheep and cattle livers and kidneys. Lower activities were found in cattle, indicating that factors other than biotransformation are responsible for the clincial tolerance of cattle to B. decumbens toxicity.