The regional distribution of kassinin-like immunoreactivity (KLI) was investigated by radioimmunoassay in central and peripheral tissues of the cat. In the cat brain KLI was found to have a widespread distribution with the highest concentrations present in the substantia nigra followed by the hypothalamus and caudate nucleus. Moderate levels were detected in the spinal cord, brainstem and thalamus but only low levels were found in the frontal cortex and cerebellum. In the periphery KLI was present in moderate amounts in the pituitary but in only low amounts in the dorsal root ganglion. KLI was present throughout the gastrointestinal tract with the highest concentration in the duodenum. The distribution of KLI was found to resemble closely that of substance P-like immunoreactivity (SPLI) although the ratio of SPLI to KLI varied widely throughout the tissues tested. In the frontal cortex and cerebellum the concentration of SPLI was at least 5-fold higher than that of KLI whereas in the pituitary gland, caudate nucleus and terminal ileum the concentrations of SPLI were only 50% higher. High pressure liquid chromatographic analysis of individual tissue extracts was used in order to characterize the KLI. In the caudate nucleus, substantia nigra and hypothalamus the major immunoreactive peak co-eluted with synthetic substance K. In contrast, in the spinal cord and terminal ileum the major peak of KLI co-eluted with synthetic neuromedin K. These results demonstrate that KLI is widely distributed throughout cat tissues and that this distribution closely resembles that of SPLI. However, it also appears that there are important qualitative differences in the KLI between the areas tested.