The distribution of axons and axon varicosities on the surface of the mouse vas deferens has been determined following fluorescence of these structures with 3,3-diethyloxardicarbocyanide (DiOC2 (5)) to locate varicosities and FAGLU to detect catecholamine containing nerves. Small bundles of fluorescent axons treated with DiOC2(5), were shown to give rise to single or very small bundles of 2 or 3 varicose axons that passed over the surface of the muscle bundles. Varicosities had the same average diameter of 0.9 microns, length of 1.1 micron and spacing apart of 4.6 microns whether identified following DiOC2(5) fluorescence or the FAGLU method for catecholamines and this was shown statistically to imply that they came from the same population of varicosities. Serial thin sections through small axon bundles and single axons, viewed with the electron microscope, confirmed the dimensions of varicosities along axons observed following DiOC2(5) staining or use of the FAGLU method.