The thiamine requirement of young guinea pigs and thiamine stability in semipurified diets for guinea pigs were investigated. The minimal level of thiamine·HCl supplementation which supported maximal gains, performance, and survival was in the range of 2.0 to 2.3 mg/kg of diet, depending upon the level of salt mixture. Acutely deficient animals showed poor growth, rough hair coat, anorexia, reduced food consumption, watery eyes, diarrhea, head retraction, unsteady gait, hyperextended hind legs, slobbering, and eventually, death. Symptoms were noted after feeding the deficient diet for about 2 weeks. Control animals fed thiamine·HCl had none of these symptoms. To minimize destruction of thiamine in the experimental diet, a new salt mixture, designed specifically for guinea pigs fed casein diets, was formulated and used successfully at 6% and 9% of the diet. At the 6% level thiamine stability in storage at 4° was optimal. Several other salt mixtures commonly used in guinea pig diets were compared for destructive effects on thiamine stability in refrigerated diets and all were found more destructive than the newly formulated mixture.