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The major organs of the male urogenital (UG) system have been examined in various mammals, including man, using light and electron microscopic (EM) histochemical methods. For the light microscopic study, the urinary bladder, the vas deferens and the penis (corpora cavernosa and corpus spongiosum) were studied in the rat, cat, dog, monkey and man using a glyoxylic acid (GA) method modified for peripheral adrenergic nerve fibers, and a thiocholine method for acetylcholinesterase (AChE). Fine structural analysis was done on the vasa of rat, cat, monkey and man, and on the bladder and penis of cat, dog, monkey and man. Tissue was fixed in glutaraldehyde (GMO) as a control or in glutaraldehyde-dichromate (GDC) for the specific localization of norepinephrine (NE). All organs studied demonstrated numerous adrenergic nerve fibers throughout the muscular layers, in the connective tissue, and in the adventitia of most blood vessels. These fibers had a brilliant fluorescence when visualized with the GA method, and demonstrated many varicosities with small (400-600 A) and/or large (800-1200 A) granular vesicles in both control and GDC-fixed tissue examined with the EM. Evaluation of the vesicles with the analytical electron microscope (AEM) verified that those in the GDC-fixed tissue were chrome-positive, and, therefore, NE-containing. In the vas and penis, acetylcholinesterase(AChE)-positive nerve fibers were encountered less frequently at the light microscopic level than adrenergic fibers, and few typical cholinergic varicosities were seen in these organs with the EM. In the bladder, cholinergic nerves were seen with about the same frequency as adrenergic fibers in both light microscopic and EM preparations. Also observed frequently in each of the viscera were varicosities with large to very large (800-2000 A) granular vesicles of the kind presently hypothesized to be peptidergic or purinergic. Few varicosities of the type considered sensory, with large (800-1200 A) clear vesicles and numerous mitochondria, were observed in this tissue. Evidence from this study suggests that mammalian UG organs are innervated extensively by adrenergic nerves, and, excepting the bladder, have a limited cholinergic innervation; in the bladder, numerous fibers of each type can be found. In addition, another type of nerve fiber, perhaps peptidergic or purinergic, is found in large numbers in each of the organs studied and thus may represent a significant effector of autonomic regulation.