It has been well documented that unlike in vitrification during slow cooling at least two major factors, dehydration and ice crystal formation and their growth affect the survival rate of cryopreserved plant material. Presented results with shoot tips of medicinal plant species Hypericum perforatum show negative correlation between cooling rate and recovery after cryopreservation with a maximum of 34.4% at 0.3 degrees C/min. Dehydration decreases the water content in plant cells and tissues via prolonged exposure time to cryoprotective solutions and/or increased water reduction rate, both at lower cooling rates. Our results indicate no significant influence of cooling-induced dehydration on regeneration capability in samples cooled at different rates. The thermal gradients in the cryo-tube generated during the cooling process represent one of the key parameters of resulting solution's thermal behavior. Differential thermal analysis measurements confirm positive correlation between "steepness" of thermal gradients and cooling rates. (c) 2010 Elsevier B.V. All rights reserved.