In the rat isolated urinary bladder, exposure to capsaicin (1 microM) produced a contraction thought to involve neuropeptide(s) release from sensory nerves. A second application of the drug had no motor effect indicating desensitization. The establishment of the desensitized state requires the presence of extracellular calcium. In the presence of Ruthenium red 9RR, 30 nM-10 microM) the first response to capsaicin was reduced and a concentration-dependent protection from capsaicin desensitization was observed. RR up to 10 microM had no inhibitory effect toward contractions produced by exogenous substance P nor by electrical stimulation of efferent nerves.