15% of L-3-(beta-hydroxy-alpha-methyl-phenethyl-amino)-3'-methoxy-propiophenone-hydrochloride (oxyfedrine [OF]; ildamen¿) are degradated in Krebs-Henseleit solution under conditions which are necessary for recording a complete concentration-effect curve on guinea-pig papillary muscle (total time max. 100 min). Norephedrine and 3-methoxyacrylophenone are qualitatively analysed as products of degradation by thin-layer chromatography (TLC). Secondary reaction products of the primary degradation are not detectable under these conditions. The quantitative analysis of oxyfedrine, norephedrine and 3-methoxyacrylophenone was carried out by spectrophotometry. Comparatively oxyfedrine and norephedrine were analysed by direct quantitative TLC (measuring of reflectance). The same results were obtained, in principle, by both methods. The importance of this relatively slow degradation for pharmacological in vitro experiments on the mode of action of oxyfedrine is pointed out.