1. Four pregnant sows were used to study lysine requirements by means of the interpretation of values for plasma amino acid and blood urea. Diets containing graded levels of dietary lysine were fed at the rate of 1.82 kg/d. 2. Plasma lysine remained at a low level up to 6.4 g dietary lysine/kg (dry matter basis) and then increased sharply with further increases in lysine intake. 3. Blood urea levels decreased when the dietary lysine content was increased to 6.4 g/kg and then increased at higher dietary lysine concentrations. 4. Both criteria of response indicated that the lysine requirement of the pregnant sow given 1.82 kg diet/d during the later stages of pregnancy does not exceed 10.00 g/d.