A landfill in Florida which received wastes in 1968 and 1969 has contaminated nearby shallow ground water. Analysis of a sample of contaminated ground water collected in 1984 from a well approximately 300 m from the landfill tentatively indicated the presence of the addictive sedative pentobarbital, among many other compounds. A well at the same location was resampled in 1991, and pentobarbital was positively identified using gas chromatography/tandem mass spectrometry. The persistence over 21 years of a supposedly unstable drug in anoxic ground water raises the possibility that other pharmacologically active compounds may be found in plumes coming from landfills that have accepted medical wastes.