Rats received for 6 consecutive days a diet containing 20% of the protein under test, or an equivalent amount of amino acid mixture. Protein synthesis of liver ribosomes was determined by measuring the incorporation of 14C-labelled amino acids into protein in vitro. Protein synthesis was related to (a) ribosomal RNA (mg), (b) wet weight of liver (g), (c) total liver wet weight, and (d) liver per 100 g body weight. The following five nitrogen sources were studied. Preparations of the microalgae Scenedesmus obliquus, Spirulina platensis; wheat gluten, amino acid mixtures deficient in either lysine or methionine. When compared to a standard diet containing a high-quality protein (casein supplemented with methionine), or a complete amino acid mixture, the five nitrogen sources gave relative reductions in protein synthesis. (1) The primary effect was on activity per unit of ribosomal RNA. This was obtained by giving rats one of the following preparations; Scenedesmus obliquus, Spirulina platensis, wheat gluten, or an amino acid mixture deficient in lysine or methionine. (2) Due to a decrease in liver weight, an effect on activity per total liver wet weight was observed after feeding Spirulina platensis, wheat gluten or the amino acid-deficient diets. (3) A smaller number of ribosomes are responsible for a lower protein synthesis per gram liver after feeding wheat gluten, or the amino acid-deficient diets. A less pronounced change was obtained with Spirulina platensis. (4) Liver activity related to 100 g body weight was lower with wheat gluten, the amino acid-deficient diets and Spirulina platensis. Activity was restored within 1 day by giving a complete amino acid mixture after 6 days of lysine deficiency.