Several reports have suggested that human semen quality has declined throughout the world over the last few decades. Chemicals in the environment acting as endocrine disrupters have been implicated as a possible cause. If this is indeed the case, then similar effects may be observed in animals. We analyzed 1489 ejaculates collected from 390 Breton draught stallions between 1981 and 1996. Semen was collected from all the stallions at a single center, according to standardized semen collection protocols and laboratory methods. Semen volume decreased slightly but significantly and there was an increase in sperm concentration over the study period. However, total sperm production was unchanged. Seminal fluid volume is controlled by accessory sex glands, which are regulated by androgens. Chemicals with antiandrogenic properties have been detected in the environment. By affecting the development or function of accessory sex glands, these chemicals may be at least partly responsible for the observed decrease in semen volume.