An examination has been made of the significance of several vitamin factors in the production of bracken ‘poisoning’ in ruminants. Bracken feeding led to a rapid cessation of urinary vitamin‐B1 excretion but this was not associated with any consistent increase in pyruvic acid excretion or in blood pyruvic acid concentration. Injections of vitamin B1 given to ruminants showing symptoms of bracken ‘poisoning’ did not produce any recovery. Although bracken‐fed ruminants probably have a subnormal vitamin‐B1 status, no evidence was obtained of any acute deficiency. Measurements of blood prothrombin time suggested that no deficiency of vitamin K is involved in the production of this disease, but low body‐reserves of vitamins A and C were found in experimental sheep which had died with bracken ‘poisoning’.