Two selected hairy root lines of Datura, D. innoxia and D. candida x D. aurea were compared for their performance in growth and tropane alkaloid productivity in shake flasks. Specific consumption rate and biomass yield on sucrose and nitrates during the phase of maximum growth were calculated versus changes in liquid medium volume during culture. Growth parameters were found to be better in D. innoxia hairy root cultures than in D. candida x D. aurea. Overall productivity of hyoscyamine in D. innoxia (2.1 mg l-1 d-1) was double that in D. candida x D. aurea (1 mg l-1 d-1). However, hybrid transgenic roots produced 0.5 mg l-1 d-1 of scopolamine. The relationship between water uptake and root growth was studied. During the first days of culture, water was released into the culture medium and then reabsorbed in the root tissue. After that, water uptake on a biomass basis in D. innoxia and D. candida x D. aurea transformed root cultures reached 0.74 and 0.53 ml g-1 FW, respectively. The correlation between biomass accumulation and liquid medium volume could be used as a simple and inexpensive method for indirect estimation of root growth.