The effects of biotin deprivation on various plasma constituents were studied in male Sprague-Dawley rats, some of which were also treated with sodium nitrate (NaNO2), extra niacin or linoleate. A basal diet containing 15% pork fat (lard), 30% egg white and sucrose, was fed for 7 weeks, with or without a weekly supplement of 150 microgram biotin and the other substances mentioned. Biotin-deprived rats ate significantly less food, gained less weight and had lower food efficiency ratios than biotin-supplemented rats. Rats given NaNO2 in the drinking water had significantly higher levels of methemoglobin than those without it; values were highest when extra niacin or linoleate was added to the diet. Lactate was highest in deprived rats and was lowest in rats supplemented with biotin alone. Blood lipids were higher with than without biotin, regardless of NaNO2. Levels of phospholipids and triglycerides were highest when extra niacin was added to the diet. Further studies are needed to define the role of biotin in oxidation-reduction reactions and in regulation of lipid metabolism.