The objective of this paper is to discuss recent developments in the application of biological markers to animal models of male reproductive toxicology. We have divided this paper into three major sections: First, a discussion of the testing protocols currently under investigation by the National Toxicology Program (NTP) and the EPA's Health Effects Research Laboratory; second, an examination of what we consider to be important and practical biological markers available to investigators to assess the effects of toxicants on the male reproductive system; and third, a discussion of promising new technologies, such as molecular and immunological probes, and in vitro techniques using isolated and cultured cells, which in the future may be exploited for the development of additional biological markers of male reproductive toxicity. Where appropriate we have made specific recommendations for the use of these biological markers in animal protocols and have pointed out those noninvasive markers which have application to screening human males.