The rapid trap closure of Dionaea muscinula Ellis has been explained by either a loss of turgor pressure of the upper epidermis, which should thus become flexible, or by a sudden acid-induced wall loosening of the motor cells. According to our experiments both explanations are doubtful. Objections against the turgor mechanism come from the determination by extracellular measurements from the upper epidermis of action-potential amplitudes before and after trap closure. Neither time course nor amplitude of the action potentials are altered by trap closure. In contrast a rise in the apoplastic concentration of K(+) or Na(+), which are the only ions present in the trap in osmotically significant concentrations, from 1 to 10 mM reduces the action-potential amplitudes by 25% and 15%, respectively. Furthermore, after trap closure the upper epidermal cells retain a considerable cell sap osmolality of 0.41 mol·kg(-1) which equals that of the mesophyll cells as determined by incipient plasmolysis. A sudden cell-wall acidification causing movement is improbable since an acidification of the apoplast from pH 6 to pH 4 reduces action-potential amplitudes by 33% whereas the amplitudes measured extracellylarly from the mesophyll and lower epidermis remain unchanged by trap closure. In addition, buffering the apoplast at pH 6 does not prevent movement in traps which have been incised several times from the margin to the midrib to facilitate buffer diffusion into the mesophyll. Even an alkalinization of cell walls of plasmolysed leaf segments to pH 9 does not prevent considerable extensions of the mesophyll and subsequent movement of the specimens during deplasmolysis.These experiments make it very likely that the mesophyll cells are already extensible but are kept compressed in the open trap, thus developing tissue tension. The mechanism which prevents their extension as long as the trap is open can so far only be explained for traps which have been paralysed by a long-term incubation in 1 mM La(3+). Leaf strips taken from stimulated, closed traps, comprising the lower epidermis and some mesophyll, prove to be highly extensible if they are stretched perpendicular to the midrib on a constant-load extensiometer. By contrast, strips taken from the lower side of paralysed traps are as rigid as those from the upper side of both stimulated and paralysed traps. From observations of semithin cross sections in a polarizing microscope, it is concluded that the extensibilities of these tissue strips are mainly determined by the cell walls of the upper epidermis plus a layer of adjacent mesophyll and by the lower epidermis, respectively, since these are the only cell walls with a preferential microfibril orientation in the direction of the applied stress.