Male Wistar rats six month old were fed on a diet containing 10% casein heated for 4 h at 140 degrees C; control rats got untreated casein. The content of available lysine (as determined according to RAO et al.) was 5.8 g/16g N in heated and 8.5 g/16 gN in untreated casein. The contents of free and total lysine and of epsilon-DNP-lysine were estimated in deproteinized portal blood plasma and in urine. In the former the contents of the remaining amino acids before and after acid hydrolysis were also determined. Deproteinized portal blood plasma of rats fed on untreated or heated casein contained, respectively, un mumoles per 10 ml: total lysine 9.0 and 9.9; free lysine 3.7 and 2.8; bound lysine (total lysine -free lysine) 5.3 and 7.1; peptide-bound lysine (determined as epsilon-DNP-lysine) 4.9 and 4.6; and epsilon-bound lysine (bound lysine - peptide-bound lysine) 0.5 and 2.6. The data indicate that lysine with blocked epsilon-amino group was released in the course of digestion of the heated casein and absorbed. This is additionally substantiated by the presence of this form of lysine in the urine of rats given heated casein.