Studies of blood-brain barrier (BBB) permeability to peptides are subject to several possible artifacts. Methods which inject tracer intravenously or arterially are preferred. Intraperitoneal and subcutaneous injection should be avoided. If appearance of systemically injected radiolabeled peptide in brain is observed, chromatographic verification of brain is necessary to assure the radiotracer's identity to draw conclusions about the penetration of the parent peptide. 14C and 3H peptides cannot achieve a sufficiently high specific activity to allow their use in the study of carrier-mediated peptide BBB penetration because the probable half-saturation concentration of such carriers would be below any attainable injection concentration. Studies of very high specific activity, 125I-cholecystokinin and 125I-somatostatin, reported here revealed no significant brain retention after single circulatory passage following arterial injection in the rat. This offers support to the lack of carrier-mediated BBB penetration of these peptides in the concentrations injected. These studies do not preclude a direct brain effect mediated through regions of the brain which normally lack a BBB.