A high pressure liquid chromatographic isocratic procedure is described for determining and quantitating the 5 major alkaloids narcotine, papaverine, thebaine, codeine, and morphine in Papaver somniferum L. and thebaine in Papaver bracteatum Lindl. Other papaveraceous alkaloids, including salutaridine, oripavine, laudanosine, isothebaine, cryptopine, alpinigenine, narceine, protopine, and gnoscopine, were also quantitated. The values for morphine, codeine, and thebaine in P. somniferum were in agreement within 5--9% with values obtained by the United Nations Narcotics Laboratory by other methods. In contrast to previously reported procedures, the advantage of this method is that no precolumn or other purification other than solvent extraction of the capsular tissue is necessary. Isocratic chromatography alone on a single column resolved the 5 major alkaloids.