From Papaver pseudocanescens M. PoP. mecambridine, alkaloid P0-5, protopine, cryptopine, amurensine, amurensinine and a new quaternary alkaloid mecambridine methohydroxide have been isolated. Papaver pseudocanescens M. PoP. is a perennial septentrional species of poppy of the Scapiflora section. Nothing was known until recently of the alkaloids of this plant except for the presence of papaverrubines B, D and E. Prior to the termination of our present work Pfeifer and Thomas published a paper describing the isolation of mecambridine (oreophiline), alkaloid P0-5 (alborine), protopine, amurensine, papaverrubines C, D, and E and proving the presence of traces of rhoeadine. The plants grown in our country were characterised by a relatively very low content of total alkaloids (0·02%). In accordance with the literature we also separated mecambridine as the main alkaloid. This alkaloid was isolated first from Meconopsis cambrica (L.) Vm. and it was found later in P. oreophilum F. J. RUPR., and in a larger number of other Papaver species of the sections Pilosa, Scapiflora, and Macrantha. The originally proposed structure was later revised and formula I proposed. We isolated from the mother liquors after mecambridine a small amount of a strongly polar alkaloid in the form of its iodide, which was found identical with the iodide of alkaloid P0-5 ("alborine") (refs 9, 10). It seems probable that this substance is an artifact formed on oxidation of mecambridine with air oxygen. We also isolated in low yields amurensine (Ilia), protopine and two additional alkaloids, amurensinine (IIIb) and cryptopine, which had not been found in this plant so far. In addition to this we also proved the presence of trace amounts of not identified papaverrubines. On the other hand, we were unable to prove the presence of even traces of rhoeadine. From the fraction of strongly polar alkaloids we isolated a quaternary base in the form of its iodide which according to IR and UV spectra, melting point determination and mixed melting point and optical rotation value was identical with (-)-mecambridine methiodide (IV). This is the first case of the occurrence of this alkaloid as a natural substance.