Twelve mature Yorkshire x Landrace sows were used in two separate 6 x 6 Latin-square experiments to determine the total aromatic amino acid (TAAA) and phenylalanine (Phe) requirements during lactation. A control diet of corn sugar, cornstarch, whey, L-glutamic acid, Solka floc, soybean oil, amino acids, minerals and vitamins was supplemented with L-Phe to provide .30, .45, .60, .75, .90 and 1.05% TAAA in the first experiment and .175, .250, .325, .400, .475 and .550% Phe in the second. All diets contained .09% tyrosine (Tyr) in the TAAA study and .47% Tyr in the Phe study. Diets were fed to a maximum of 5.5 kg/d during 10-d (TAAA study) and 7-d (Phe study) lactational periods;In the first experiment, feed intake varied among diets (quartic, P < .05) and sow milk yield was maximized at .75% TAAA. Urea nitrogen (N) decreased quadratically with increasing dietary TAAA in both plasma (P < .05) and urine (P < .01) to .56% TAAA. Plasma Phe increased quadratically (P < .01) without clear inflection point and plasma Tyr increased sharply at .73% TAAA. Plasma lysine reached a lower plateau at .76% TAAA. Urine N excretion decreased (quadratic, P < .01) and N retention increased (quadratic, P < .01) to .58% TAAA;In the second experiment, feed intake and average pig weight gain were lower (quadratic, P < .05) for the lowest Phe diet. Percentage urine N decreased (quadratic, P < .001) and percentage N retained increased (quadratic, P < .001) with increasing Phe to .288 and .296% Phe, respectively. Percentage overall N balance was maximized at .307% Phe. Urea N decreased with increasing Phe in both plasma (linear, P < .01) and urine (quadratic, P < .001) to .285% Phe. Plasma Phe rose sharply (quadratic, P < .01) at .299% dietary Phe, but there was no clear inflection point in plasma Tyr. Plasma lysine and leucine decreased, and plasma isoleucine and valine increased with increasing Phe to .317, .303, .367 and .359% Phe, respectively;With equal weight given to the N balance and plasma amino acid response criteria, .65% TAAA with .30% Phe would seem to meet the sows' dietary requirements. This would increase to .75% (41.2 g/d) TAAA and .34% (18.7 g/d) Phe for sows fed 5.5 kg/d of a diet containing natural ingredients.