Reproductive risks associated with exposure of human males to various toxic agents were reviewed. Sites of toxicant action, including the neuroendocrine system, the testes, accessory sex glands, and the integrated activities of the testes, secondary sex glands, endocrine control systems, and the central nervous system based behavioral and psychological components of reproduction, were described. Procedures for assessing the function of each of these sites were reviewed. The contribution of environmental toxicant exposures, life style practices, and occupation to male reproductive health and functioning was described. The use of population based, case control, cohort, and clinical studies, as well as standardized fertility ratio determinations for assessment of the effects of occupational exposures to toxins on the reproductive system was explored. Difficulties associated with developing surveillance strategies for the evaluation of workers with occupational exposure to known male reproductive toxicants were discussed. These included differential susceptibilities of subgroups of individuals and the role and accuracy of reports of declining sperm counts. The author concludes that a team approach is most useful for the complex evaluation of male fecundity.