Electrophysiological techniques were used to examine the asynchronous and evoked activity on postganglionic nerves to the urinary bladder in the urethananesthetized rat. Distension of the bladder (0.4-0.6 ml) evoked reflex contractions of the bladder (mean intravesical pressure 28 cmH2O) and efferent firing on postganglionic nerves. Electrical stimulation of afferent and efferent axons in the pelvic nerve elicited short-latency (0.3-11 ms) responses and long-latency (45-170 ms) reflexes on the nerves. The short-latency responses consisted of nonsynaptic axonal volleys with conduction velocities ranging from 0.5 to 11 m/s and synaptic responses with latencies of 6-11 ms. Stimulation of the pelvic nerve elicited late supraspinal reflexes (mean latency 122 +/- 28 ms) in 60% of normal rats and an early reflex (mean latency 56 +/- 5 ms) in 25% of those animals in which a late reflex was also identified. Early reflexes (mean latency 50 +/- 9 ms) were elicited in 100% of chronic spinal animals. The conduction time for the afferent and efferent limbs of the reflexes was calculated to be 7 and 58 ms, respectively, with a central delay of 57 ms for the late and less than 5 ms for the early reflex. It is concluded that sacral parasympathetic input to the urinary bladder of the rat is mediated by supraspinal and spinal reflex pathways. It is likely that in normal animals the late-occurring supraspinal reflex mediates micturition. The significance of the spinal reflex in the normal animals is uncertain; however, this reflex is essential for the generation of automatic micturition in chronic spinal preparations.