A rapid and sensitive HPLC method has been developed for the determination of codeine, norcodeine and morphine in small volumes of a biological matrix, using a cyanopropyl column and a combination of coulometric and UV detection. The compounds were isolated using C18 solid-phase extraction cartridges prior to quantitative analysis. The limit of detection was 250 pg/ml for morphine and 5 ng/ml for both norcodeine and codeine. Recovery of each compound was greater than 90% and intra- and inter-assay precision was better than 10%. The method has been used to study the metabolism of codeine in microsomal incubations.