1. The effects of haemorrhage on vitamin B12 (cobalamin) homoeostasis have been studied in rats with and without prior administration of 57Co-labelled cyanocobalamin.
2. Arteriovenous differences in radioactivity and endogenous B12 were measured across the thigh extensor muscles. Radioactivity and endogenous B12 were also measured in skeletal muscle, liver and kidney.
3. Haemorrhage induced a marked rise in circulating B12 with a concomitant loss of the vitamin from skeletal muscle and liver, but in kidney there was a net gain. Restoration of the blood volume with autologous plasma did not prevent release of B12 from tissues.
4. The results suggest that vitamin B12 reserves may be seriously depleted by the wasting of skeletal muscle.