To clarify the influence of protein calorie malnutrition (PCM) on thiamin metabolism in rats, the effects of three diets containing different amounts of protein and sucrose were evaluated by three tests in 20 young male Wistar rats equally divided into four groups. Three of the groups, the controls (group 1), those fed high protein (group 2), and one of the two groups fed a low protein diet (group 3), were fed ad libitum. A second group (group 4) on the same low protein diet was fed 40% less than that consumed by group 3, which consumed less than the controls or high protein rats because of poor appetite. The sucrose contents of the experimental diets were inversely related to the protein contents. Thiamin concentrations were determined in blood, brain, heart, liver, kidney and spleen, and in 24-hr urine and stools. Tissue transketolase activity (TTKA) was measured in brain and liver and in erythrocytes (ETKA), the latter in association with the thiamin pyrophosphate (TPP) effect. Malnutrition caused decreased 24-hr urinary thiamin excretion, decreased ETKA, with corresponding increased TPP effect and decreased TKA of brain. As tissue thiamin concentrations were not decreased, the results suggest that malnutrition causes functional thiamin deficiency.