Previous studies have demonstrated neuroprotective effects of the opioid peptide dynorphin (dyn) 1-13 in focal cerebral ischemia. The passage of dyn 1-13 across the blood-brain barrier (BBB) was studied by a modification of the Oldendorf technique in the normal rat and cat, as well as in a feline model of experimentally induced focal cerebral ischemia. In the rat, dyn 1-13 penetration of the BBB could not be detected by this technique, even in the presence of peptidase inhibitors. In contrast, dyn 1-13 did cross the BBB into the normal cat hippocampus, cortex and cerebellum. The passage of dyn 1-13 across the BBB was greater in cats with experimentally induced focal cerebral ischemia. Some of the tritium-labeled material which crossed the BBB was confirmed by high performance liquid chromatography to be dyn 1-13. These studies support the hypothesis that the therapeutic effects observed after the peripheral administration of dyn 1-13 to cats with focal cerebral ischemia can be produced by a central mechanism of action.