This review will begin with a brief account of the secretory mechanisms operating in the male reproductive tract. The entry of the different chemicals will be dealt with; with separate reference to testicular fluids, epididymal plasma, prostatic and vesicular secretions, and whole semen. Examples will be given of a number of chemical substances capable of passing into the secretions of the male reproductive tract and semen, in man and in animals--antispermatogenic and antiandrogenic agents, industrial chemicals, chemotherapeutic drugs, food additives, etc. The review will end with a critical appraisal of the methods involved and the results obtained from analyses of various chemicals in semen.