The influence of culture age on biomass production and alkaloid yield of "hairy roots" obtained after infection of Datura stramonium L. with Agrobacterium ATCC 15834 was investigated. Maximal hyoscyamine yield was obtained with roots harvested after six weeks. Fluctuations were found for tropine yield, the precursor of the ring moiety of hyoscyamine. These indicate a continuous conversion to hyoscyamine during the exponential growth phase.The effect of the ion-balance was investigated by preparing five different media that only differed in their ionic composition. The ionic interactions between macroelements, differently influenced biomass production and alkaloid yield. As a result, highest biomass yield was found with NO 3-and K +-dominance, whereas hyoscyamine 2 + yield was highest with the culture medium in which SO 4-and K were dominant. Shifting the intercationic balance to strongly towards Ca 2+ caused an overall reduced metabolism, since as well biomass yield as hyoscyamine yield was lowest with the NO3-Ca2+-medium. Also tropine yield was affected by the ion-balance, indicating that this culture parameter also influences alkaloid synthesis.Abbreviations: NRA -nitrate reductase activity