L-threonine was added to a fortified corn-gelatin diet to attain theonine levels of 3.59, 4.95, 6.31, 7.67 and 9.03 g per day. These diets were fed to twenty-five crossbred gilts at a rate of 1.82 kg per day before mating and during 3 successive pregnancies. During lactation, all animals received the same diet which contained 0.35% theonine. Litter size and feed intake were standardized during both lactations;Five day nitrogen balance trials were initiated on the 45th and 90th day of pregnancy. Increasing the dietary threonine resulted in a significant difference in nitrogen retention, with maximum retention occurring when 4.95 g of threonine were fed. After each balance trial blood samples were drawn before and following feeding. The lowest level of plasma urea nitrogen occurred when 4.95 g of threonine were fed. As threonine intake increased, plasma threonine increased linearly (P < .005) and quadratically (P < .05) with the response curve inflection at 4.95 g. This increase in plasma threonine was accompanied by a significant linear (P < .01) and quadratic (P < .005) decrease in fasted plasma lysine and a significant linear (P < .05) increase in the liver sample activities of lysine's catabolic enzyme, lysine-(alpha)-ketoglutarate reductase;Sow weight gains increased linearly (P < .01) and quadratically (P < .01) with increasing theronine levels. Maximum gestation gains occurred when 6.31 g of threonine were fed, while maximum gains for the entire experiment occurred when 7.67 g were fed;Litter weight, number of pigs born, baby pig gains, daily milk yield and milk protein were not significantly influenced by threonine levels;All sows were slaughtered at 27 (+OR-) 4 days of the third gestation period. There were no significant differences in live weight, hot carcass weight, loin eye area, carcass backfat and percent muscle;The changes in metabolic criteria lead to the conclusion that a daily intake of 4.95 g of L-threonine meets the requirement for reproduction;If it is assumed that the added L-threonine was 100% digestible and that only 75.0% of the threonine present in a typical corn-soybean meal diet is absorbed by the sow, then 5.4 g of threonine per day when supplied by these feedstuffs would meet the requirement for reproduction.