Weanling albino rats were fed, ad libitum, one of three low iodine diets, or a commercial laboratory chow, for 7 weeks; then the weight and iodine content of their thyroid glands were determined. Rats fed the standard Remington (corn and wheat gluten) goitrogenic diet slowly and developed goiters attributable to iodine deficiency. Rats fed a semipurified diet containing 20% of casein grew well and developed goiters smaller than those seen in rats fed the Remington diet. Animals fed a diet containing 40% of brewers type dry yeast and 0.5% of DL-methionine grew as rapidly as rats fed the casein diet or a laboratory chow; but developed goiters significantly larger than those seen in animals fed the Remington diet. Hence the 40% dry yeast diet appeared to have a positive goitrogenic effect over and above that attributable to its low iodine content.