1. Electrophysiological techniques were used to study the sacral para-sympathetic input to pelvic ganglia located on the surface of the urinary bladder of the cat. 2. Synaptic transmission in pelvic ganglia was mediated primarily via nicotinic receptors although muscarinic excitatory receptors were present. 3. The most prominent characteristic of transmission in pelvic ganglia was the marked recruitment elicited by increasing frequencies of preganglionic nerve stimulation. Post-ganglionic action potentials were of low amplitude at low frequencies of stimulation (0-1-0-5c/s), but commonly increased to five to twenty times control amplitudes during continuous stimulation at frequencies between 5 and 10c/s. Thus, it is proposed that vesical ganglia may act as "filters" in the micturition pathway; blocking the excitatory input to the bladder when intravesical pressure and parasympathetic firing is low and facilitating the neural input to the bladder during micturition when preganglionic activity is high. 4. Information was also obtained about the characteristics of the parasympathetic post-ganglionic neurones innervating the bladder. Stimulation of the preganglionic fibres in the pelvic nerve elicited a bimodal contraction consisting of an initial phasic response, which was atropine-resistant and a tonic response which was blocked by atropine. This suggests that two types of neurones, cholinergic and non-cholinergic, may mediate the sacral input to the vesical smooth muscle.