1. Reflex effects of cold stimulation of the lower urinary tract were studied in cats anaesthetized with alpha-chloralose. The bladder and the urethra were catheterized for separate fluid instillations and the bladder pressure was monitored together with the evoked efferent nerve responses in pelvic nerve filaments. 2. A bladder cooling reflex could be evoked from both the bladder and the urethra. The response was an efferent discharge in preganglionic pelvic motor fibres to the bladder. 3. Bladder mechanoreceptors that drive the normal micturition reflex were not directly involved in the cooling reflex. Their tension sensitivity was decreased by cooling and the efferent reflex response typically occurred before any activation of these receptors. The efferent activity of the cooling reflex also survived an intentional unloading of the mechanoreceptors, a manipulation that abolishes the normal micturition reflex. 4. The dynamic threshold temperature of the cooling reflex was about 30-32 degrees C, which was at the thermal neutral point of the bladder in our experimental situation. 5. The bladder-evoked component of the reflex was greatly reduced or abolished by an intravesical infusion of the local anaesthetic Xylocaine. It was also abolished by total bladder denervation. 6. The vesical component of the reflex was unchanged by bilateral transections of the hypogastric nerves but abolished by pelvic nerve transection. The cooling reflex from the distal urethra was abolished by transection of the pudendal nerves. 7. It was proposed that the cooling reflex originates from cold receptors in the bladder and urethral walls and that the responsible afferent fibres are unmyelinated C fibres. The function of the reflex may be to rid the body of a thermal ballast when under cooling stress.