SUMMARY White Carneau pigeons are significantlymore susceptible to thiamine-deficiencythan are pigeons of the Show Racer breed.When the two breeds were force-fed athiamine-deficient diet, the levels of thiamine in the brains and livers of bothbreeds decreased, but to about the sameextent.After feeding the deficient diet for approximately 14 days, the White Carneaupigeons exhibited deficiency signs, whereas those of the Show Racer breed did not.Nonetheless, neither the level of pyruvateoxidase (as measured by increases inblood pyruvate), nor of blood transketolasediffers significantly in the two breeds.Likewise, the activity of a-ketoglutarateoxidase decreases to the same extent inthe brains and livers of both breeds ofpigeons. On the other hand, the activityof brain transketolase in either breed didnot appear to change as the deficiencyprogresses.It is postulated that the appearance ofthe neurological signs of thiamine deficiency is related to factors other than thedecreased activity of the enzyme systemsstudied here.