Some P.G.R. studies in a female subject who had bilateral cervical sympathectomy were described. It was found that sympathectomy abolished P.G.R. and that intra-arterial infusion of acetylcholine evoked marked P.G.R. changes in the sympathectomized limb. These findings support the theory that the P.G.R. is mediated through the cholinergic fibres of the sympathetic nervous system.