IN the course of studies of the metabolism of intracerebrally administered L-U-14C-histidine in brains of mice and frogs (Rana pipiens), analyses were also performed on blood samples from the injected animals. The mice received 1.5×106 c.p.m. of the labelled histidine, and 1 h later samples of blood were taken by means of orbital puncture. Frogs were injected with 2.9×106 c.p.m. and blood was obtained after 1 h by cardiac puncture. The samples were deproteinised in 5 volumes of 0.4 N perchloric acid and processed in the manner described previously for brain and tumour tissues1. Aliquots of neutralised extracts were electrophoresed on Whatman No. 3 paper at pH 10.0 together with known standards of carnosine, homocarnosine, histidine, imidazoleacetic acid, N-acetylhistidine, and histamine; and stained with Fast Red B in the manner described previously1.