Abstract Some characteristic features of the functional innervation of guinea-pig vas deferens have been determined. Both ganglionic transmission from the hypogastric nerves and impulse propagation in proximal regions (main branch bundles within about 15 mm from the prostatic end of the organ) of the majority of single postganglionic sympathetic fibres of vas deferens nerve, had a high safety factor. Failure at these levels cannot account for the intermittent pattern of electrically-evoked secretion of transmitter from the individual varicosity of the terminals of vas deferens nerves, observed under identical experimental conditions. The shape of the extracellular single fibre action potential recorded by small calibre suction electrodes remained constant in proximal regions of vas deferens nerve, when the frequency of stimulation was varied between 0.5 and 8 Hz. Therefore, frequency-dependent facilitation of transmitter secretion in this tissue cannot be explained by frequency-dependent growth in the amplitude of nerve action potentials, as earlier assumed. However, when recordings were made in distal regions of vas deferens nerve (in small axon bundles, close to their points of insertion into the substance of the epididymal end of the organ), on two occasions fibres were found in which the safety factor for impulse conduction was low and frequency-dependent. The possibility is discussed that this feature, which was an exception in these non-terminal regions of vas deferens nerve, may be shared by the majority of fibres as they proceed distally towards the terminals. Clearly, if this is the case, intermittent failure of transmitter secretion from the individual varicosity may be due, at least in part, to intermittent failure of conduction of the nerve impulse to the terminals. Some useful qualitative information on the ionic basis of the extracellular nerve action potential, that might underlie a proximo-distal decline in the safety factor for impulse conduction in these nerves, was obtained by determining the effects on the shape of the signal, caused by varying the ionic composition of the medium (sodium, calcium), and by local addition of agents with known actions on sodium (tetrodotoxin), potassium (tetra-ethyl ammonium, 4-aminopyridine, rubidium, barium) and calcium channels (cobalt, manganese, lanthanum, nickel, D-600). By these criteria, the action potential that was shown to be a “normal” sodium-potassium spike, in proximal regions of vas deferens nerve, was found to have a different “pharmacological profile”, in distal regions of the nerve, in a manner suggesting that here nerve impulse conduction had become somehow “calcium-dependent”. Although calcium ions alone could not sustain the nerve action potential in the absence of extracellular sodium, variation in the calcium concentration of the medium altered its shape. Furthermore, impulse conduction in distal regions of vas deferens nerve was not only blocked by tetrodotoxin, but also by “calcium channel blockers” of the alkaline earth metal ion class (cobalt, manganese, nickel, lanthanum), but not by the organic “calcium blocker” D-600. In these regions 4-aminopyridine had unique effects, often causing a delayed and prolonged positive-going potential, suggesting promotion of influx of cation, possibly through “calcium channels”. The results suggest that changes in the ionic basis of the nerve impulse as the fibres approach the secretory terminals introduce an element of “plasticity of conduction”, that may be the basis of pharmacological and physiological control of transmitter secretion by regulation of the invasion of terminals by the nerve action potential.