Administration of capsaicin to newborn rats produced a slight but significant increase in bladder weight and markedly altered the cystometrograms of urethane anesthetized rats. In particular capsaicin pretreatment significantly increase micturition threshold. In many of these preparations the bladders exhibited a hypertonic response to saline filling; the flat phase of the cystometrogram was not observed. Capsaicin pretreatment of adult animals increased micturition threshold up to 1 month from capsaicin administration. In these preparations a flat volume-pressure relationship preceded micturition contractions. Administration of capsaicin to both newborn and adult animals did not alter the amplitude of micturition contractions. Intravesical installation of capsaicin had an acute excitatory effect on the responsiveness of the urinary bladder to reflex activation. These findings indicate that capsaicin pretreatment produces a functional impairment of the sensory branch of micturition reflexes and suggest that a capsaicin-sensitive mechanism(s) is involved in the regulation of micturition threshold.