The effect of feeding amino acid-supplemented diets on plasma cholesterol concentration and distribution in the lipoproteins was studied on adult rats. A control diet (without any amino acid addition), and experimental diets supplemented with one of the following L-amino acids: lysine (10%), cystine (5%), methionine (1%), tryptophan (10%), valine (5%), and histidine (5 %), were given for 2–4 months. Rats fed the lysine-, cystine- and tryptophan-added diets exhibited constant weights throughout the experiment, whereas those fed the other amino acid-added diets showed body weight gains quite similar to control rats. Two amino acids were shown to lower plasma cholesterol concentration: lysine (by 30%) and tryptophan (by 35%); one amino acid increased it: cystine (by 46%). The cholesterol distriwbution in the lipoproteins was significantly modified, principally when rats ingested cystine-enriched diets: as compared to control rats, the cholesterol concentration in lipoproteins of hensity between 1.040 and 1.063, and in high density lipoproteins (HDL), was increased by 174 and 58%, respectivel