To test the effects of an insufficient fiber content, a high percentage of readily fermentescible carbohydrate and non protein nitrogen uptake on thiamin nutrition, three goats were submitted during periods of 40 or 90 days to three diets with low fiber contents (1: 16.53%; 2: 12.37%; 3: 12.84%) and low thiamin contents (1: 0.07; 2: 0.08; 3: 0.05' mg/kg). The diets were basically composed of beet pulp and equilibrated in protein either with copra meal or with urea. Diets 2 and 3 were enriched with 20% lactose. Animals had no access to coarse forage or litter. Variations of blood transketolase activity, pyruvicemia and lactacidemia were followed. Preliminary standard values of transketolase activity were determined with a hay diet (38 +/- 10 Ul/l total blood) or a grass diet (47 +/- 13 Ul/l). In the same way, no decrease of food intake or clinical signs of thiamin deficiency could be observed. In none of the cases were noted impairment of blood transketolase activity or increase in lactacidemia or pyruvicemia. It was concluded that a diet deficient in fiber and rich in fermentescible carbohydrate cannot by itself induce a decrease in thiamin synthesis in so far as transitions between diets are not done abruptly.