The changes in the fine structure of axonal profiles of the guinea-pig ureter were examined after exposure to capsaicin in vitro. In the ureters exposed to capsaicin (1-10 microM), about 60%, of all axonal profiles exhibited conspicuous ultrastructural impairment. It is suggested that these alterations might interfere with the release of peptides from these particular nerve endings and therefore contribute significantly to the mechanisms of capsaicin desensitization.