For toxicological purposes, a HPLC assay was developed for the simultaneous determination of risperidone and 9-hydroxyrisperidone in human plasma. After a single-step liquid-liquid extraction, both compounds were separated on a C(18) column and measured at 280 nm. A good inter-assay accuracy (<116%) was achieved with inter-assay precision less than 12%. Quantification limits were 10 ng/ml. This rapid method (run time <5 min) is currently used for poison management.