A cheap simple and rapid extraction procedure followed by a UV high performance liquid chromatography (HPLC) assay is described for the simultaneous determination of morphine (M) and codeine (C) in plasma. The method is based on extraction of these opiates from plasma using reversed phase (solid phase) extractions columns followed by HPLC with UV detection at 240 nm. The extraction step provides, respectively, 85 and 80% recovery for M and C. The response of the detection system is linear for both molecules in the studied range from 50 to 750 ng ml-1. No other drugs have been found to interfere with the assay. This method offers a quick, cost effective and reliable procedure for specifically determining M and C, from a small sample volume.