Nine experiments were conducted to evaluate the potential use of plasma urea nitrogen concentration (PUN) as a rapid response criterion to determine amino acid requirements of growing and lactating pigs. A 3-d feeding time was required to re-equilibrate PUN after a change in the dietary concentration of lysine. The lysine requirements of growing pigs at different specific body weights were estimated from the PUN response to graded dietary lysine concentrations that were fed for 5 d. Increasing lysine caused PUN to decrease quadratically (P .10). Lysine requirements were estimated to be 1.05,.85,.76,.69,.75, and.66% for 25-, 36-, 49-kg pigs, 74-kg barrows, 74-kg gilts, and 108-kg gilts, respectively. The effect of a reduction in feed intake (100 and 80% of ad libitum intakes) on the lysine requirement of pigs was different (P .20) between feeding levels. Adult sows nursing 10-pig litters with an average growth of 2.2 kg/d required 55 g/d of dietary lysine to minimize PUN.