1. The possibility that a purine nucleotide is involved in excitatory transmission to the urinary bladder has been tested. All the purine compounds tested which contained a pyrophosphate bond produced contraction, adenosine triphosphate (ATP) being the most potent. Adenosine and adenosine monophosphate caused relaxation.2. The response to ATP closely mimicked the nerve-mediated contraction, both being characterized by a rapid contraction which was not maintained. A lack of sensitivity to ATP was noted in some preparations of the rat urinary bladder.3. Both nerve-mediated contractions and contractions caused by ATP were blocked by quinidine, while the response to acetylcholine persisted.4. Nerve-mediated responses were depressed during tachyphylaxis produced by high concentrations of ATP. Tachyphylaxis did not occur when low concentrations were used. Possible explanations for these results are discussed.5. The results are consistent with the hypothesis that non-cholinergic excitatory nerves to the guinea-pig bladder release a purine nucleotide, but do not provide critical evidence for it.