Plasma zinc levels in rats dropped quickly on a zinc deficient diet and stayed low throughout the study. Fasting made plasma zinc levels even lower. Whole blood zinc levels stayed normal, even after 65 days of a zinc deficient diet. Urine zinc levels dropped after 10 days on a zinc deficient diet, then rose back to normal after 30 days. Plasma zinc is a good indicator of zinc deficiency, while whole blood zinc is not useful for detecting it.