Effects of force-feeding in contrast to ad libitum-feeding diets containing 0.35, 0.50, 0.65 and 0.80% lysine on plasma free amino acids were investigated in laying hens. The feed intake and body weight gain of hens fed ad libitum were less than those in hens force-fed: this was more prominent in hens fed low-lysine diets. The concentrations of plasma arginine, histidine, lysine, methionine, phenylalanine, valine, alanine, glutamic acid, proline, serine and tyrosine were greater in hens fed ad libitum while plasma threonine, asparagine plus glutamine, ammonia and uric acid were greater in hens force-fed. Plasma lysine was increased slowly by increases of dietary lysine from 0.35 to 0.65%, but rapidly from 0.65 to 0.80%. The concentrations of plasma leucine, phenylalanine, threonine, half-cystine, taurine and uric acid were greater in hens fed a low-lysine diet (0.35%) than in hens fed a high-lysine diet (0.80%). The plasma ratio of essential to non-essential amino acids was decreased when dietary lysine was increased from 0.35 to 0.50% and 0.50 to 0.65%. These results indicated that force-feeding apparently increased the catabolic rate of dietary protein. Plasma free lysine changed in response to the dietary lysine level in laying hens.