The dietary lysine requirement of adult sows nursing 10 pigs was estimated by using plasma urea nitrogen (PUN) concentration as a rapid response criterion. A preliminary experiment indicated that a 3-d feeding time is required to re-equilibrate PUN concentrations after a change in the dietary concentration of lysine. In the main experiment, 12 sows (BW = 219 +/- 5 kg; parity = 4.5 +/- .3; ultrasound backfat = 21.3 +/- .9 mm) were used in two 6 x 6 Latin squares. Treatments consisted of dietary lysine intakes (30.2, 36.9, 43.6, 50.3, 57.0, and 63.7 g/d) that were assigned to six 4-d periods and to six sows in each square. Diets provided adequate levels of all nutrients other than lysine. The treatment periods started on d 5 of lactation and ended on d 29. Plasma urea N concentrations were determined on the 3rd and 4th d of each of the six 4-d treatment periods. Plasma urea N concentrations decreased quadratically (P < .02) with, increasing dietary lysine intakes. A two-slope, broken-line regression model was used to estimate the dietary lysine intake that minimizes PUN concentrations. The results indicated that adult sows nursing 10-pig litters with an average growth of 2.22 kg/d required 55.3 g/d of dietary total lysine to minimize PUN concentrations and, therefore, presumably to minimize body protein mobilization.