The sensory innervation of the rabbit urinary bladder was studied using local application of the specific sensory neurotoxin capsaicin (8-methyl-N-vanillyl-6-nonenamide). This agent has its major effect by damaging small diameter unmyelinated sensory nerves. The drug produced a 51% reduction in the bladder content of the neuropeptide substance P. It therefore appears that a substantial proportion of the bladder's content of the peptide is to be found in capsaicin-sensitive sensory nerves. Cystometrograms carried out before and after treatment with capsaicin were similar; this suggests that capsaicin-sensitive sensory nerves may not be of importance in the afferent limb of the micturition reflex. In vitro muscle strip studies demonstrated a small reduction in the sensitivity of the detrusor muscle to electrical stimulation of its intramural nerves. It is possible that in vitro intramural nerve stimulation leads to release of neurotransmitters from sensory as well as motor nerves. It is proposed that small diameter sensory nerves in the bladder wall may have a role in the transmission of the sensation of pain and in the triggering of inflammatory reactions rather than forming the afferent limb of the micturition reflex.