Steroids given to cattle to promote growth can wind up in water supplies, where they may act as potent endocrine disruptors in wildlife. But some of the metabolized compounds, such as those stemming from trenbolone acetate, are believed to degrade too quickly to be a problem. Not so fast, says a group led by David M. Cwiertny of the University of Iowa and Edward P. Kolodziej of the University of Nevada, Reno. In both laboratory and field experiments, they find that compounds such as 17α- and 17b-trenbolone degrade via light-induced hydroxylation during the day but convert back to the parent compound in the dark (Science 2013, DOI: 10.1126/science.1243192). The driving force behind the regeneration reaction may be to restore a conjugated trienone system in the steroid framework. Similar regeneration occurs for steroids used for oral contraception and, illegally, for bodybuilding, the researchers report. The results suggest that regeneration of steroidal contaminants should be part of environmental risk-assessment models.