We have postulated that barbiturate-induced sleeping times can be used as an indicator of microsomal enzyme activity induced by environmental contaminants in wild populations. We have demonstrated this in a wild population inhabiting an area known to be contaminated. Muskrats collected from a region of the Elizabeth River, Va. known to be contaminated with a variety of environmental pollutants (heavy metals, polynuclear aromatic hydrocarbons, DDTs, PCBs, etc.) had a mean sleeping time (36 min) that was approximately half the mean sleeping times recorded from 2 uncontaminated areas (60 and 63 min). Laboratory studies using barbiturate-induced sleeping times to indicate changed microsomal enzyme activity resulting from exposure to specific hepatotoxins, anticholinesterases, chlorinated hydrocarbons insecticides, polychlorinated biphenyls, and polynuclear aromatic hydrocarbons have been reviewed.