Viable hybrids were artificially produced between Papaver bracteatum rich in thebaine on the one side and Papaver somniferum, Papaver orientale and Papaver pseudo-orientale on the other side. Each of them is characterized by morphological and some physiological features as well as by its chromosome number. Biochemical analysis confirmed that P. somniferum-P. bracteatum hybrids contain very high morphine concentrations if the biosynthetic activity of the first parent is not to low. Also other conditions favouring the morphine yield of this hybrid could be elucidated. - Oripavine is the main alkaloid in P. orientale-P. bracteatum hybrids. However, its concentration remains low. This is probably due to the limited oripavine biosynthesis in P. orientale plants used for cross experiments. - P. bracteatum-P. pseudo-orientale hybrids contain thebaine only in trace amounts. These small quantities are characteristic of the P. pseudo-orientale parents. Interspecific hybridizations corresponding to a “genetic feeding experiment” generally result in increased concentrations of desired morphinan alkaloids. However, their effectiveness remarkably depends on the following fact: Low alkaloid levels may frequently be the expression of a reparable shortage, but they can also be caused by a strict regulation snowing dominant inheritance.