An analytical procedure that enables routine analysis for trace determination of six anabolic macrocyclic lactones (zearalenone, alpha and beta -zearalenol, zearalanone, zeranol, and taleranol) in sewage treatment plant (STP) samples has been developed. The method uses solid-phase extraction, followed by high-performance liquid chromatography with on-line tandem mass spectrometry using atmospheric pressure chemical ionization (LC/APCI-MS/MS). The extraction of these compounds from filtered water samples was performed off-line with Gs solid-phase cartridges. The detection was achieved by isocratic reversed-phase high-performance liquid chromatography coupled with an heated nebulizer (HN) APCI interface operating in negative ion mode. Mean recovery of the analytes in STP effluent samples generally exceeded 81%. This method was used to determine the occurrence of target analytes in the aquatic environment. In the selected STP effluent samples, zearalenone and alpha -zearalenol were detected in the ng/L range. Copyright (C) 2001 John Wiley & Sons, Ltd.