Physiological disturbances common to B6 avitaminosis were clearly manifested when 3-day-old chicks were fed a pyridoxine-deficient diet for 8 days. Body and liver weights were depressed but were restored to quasi-normal states after 4 days of pyridoxine supplementation (3 mg/lb diet). The elevated serum cholesterol levels observed were the result of a failure of this parameter to fall at the rate characteristic of a well-fed bird and not to a specific elevation of the serum cholesterol above the starting conditions. Liver cholesterol levels were unaffected by the hypovitaminosis. Pyridoxine supplementation rapidly re-established normal serum cholesterol levels.The significant depression of C14-acetate incorporation into liver and serum cholesterol of 7-day-old pyridoxine-deficient chicks was maintained for the next 8 days. Mevalonate-2-C14 incorporation into liver cholesterol was not significantly depressed by the hypovitaminosis at any time during the study. Unlike the controls, mevalonate-2-C14 incorporation into serum cholesterol was significantly lowered in the 11-day-old pyridoxine-deficient chicks, but not at any other time during the study.The significance of these findings and the possible relationship between these factors are discussed.