Acute intracerebroventricular injection of 25 micrograms capsaicin or 40 micrograms substance P in isotonic saline elicited approximately similar effects on the micturition reflex, but capsaicin had twice as much effect as substance P. This effect is specific, since acute intracerebroventricular injection of isotonic saline did not produce the micturition reflex. It can be hypothesized that capsaicin and substance P may act on the brain micturition centers directly or by mediation of neuropeptides such as tachykinins, but other hypotheses are also made.