Abstract Different aspects of the biochemistry of chromium in laboratory animals have been investigated using 51Cr radiotracer methods. 51Cr-labelled chromium compounds such as cationic trivalent 51Cr 3+ and anionic hexavalent 51Cr 6− were prepared and administered I.V. to rats in doses ranging from 0.1 to 100 μg/rat. The results show remarkably different metabolic patterns in the two chemical species. They refer (1) to the distribution in the blood. More than 95% of the 51Cr in the blood of rats administered with the trivalent form was present in the plasma while the corresponding value in the case of the hexavalent species was of about 25%. Gel permeation chromatography on sephadex G-150 and ion exchange chromatography in DEAE show that the 51Cr plasma of rats treated with 51Cr 3+ is associated with β-globulin transferrin. Dialysis experiments indicate the strong nature of the binding of chromium to transferrin. They also refer (2) to the biliary excretion patterns. The biliary excretion of 51Cr has been studied in the bile duct-cannulated rats 120 min after the intravenous administration of 0.1 μg Cr/rat to 100 μg Cr/rat as trivalent or hexavalent chromium. The biliary levels of 51Cr-derived radioactivity reached their peak 30 min after injection of both Cr III and Cr VI and decreased slowly thereafter. The 2 hour biliary concentrations of Cr in the animals treated with Cr VI were up to 60 times higher than those found in the rats given the same amount of the trivalent Cr form 2.6 per cent of the dose was recovered in the bile of rats injected with Cr VI within 2 hours while the 2 hours cumulative excretion of 51Cr in the bile of Cr III treated animals was approximately 50 times lower. In the first 120 min period after treatment of the bile-fistula rats the urinary excretion of 51Cr ranged from 7 to 15% of the administered dose without any obvious relation to the chemical form and the dosage level. Chromium had no effect on the rate of biliary flow. The plasma levels of 51Cr-radioactivity of the animals treated with Cr VI were significantly lower than those detected after injection of the same doses of Cr in the trivalent form. No valency-related differences were found with respect to the liver concentrations of 51Cr labelled chromium. These findings indicate that the distinctive pattern of Cr biliary excretion observed in the rat after treatment with Cr III and Cr VI are independent of the plasma-to-bile and liver-to-bile concentration gradients of this chemical species. In this study, evidence has also been obtained for a direct excretion of Cr across the intestinal wall as indicated by the presence of appreciable levels of 51Cr activity in feces in the gastrointestinal segments of rats with ligated bile duct after intravenous injection of 100 μg of 51Cr-labelled chromium. Average values of 4% and 1.5% of the injected Cr dose were measured in the gut(stomach and intestine plus contents) 24 hrs after the injection of Cr VI and Cr III, respectively. This findings provided further support for the concept that the trivalent and hexavalent forms of Cr exhibit different behaviour in their elimination patterns by the digestive tract.