The results of investigations on nonevoked bioelectrical activity in the India-rubber tree (Ficus elastica) are presented. Metal electrodes inserted into the plant issue were used as the ionic-to-electronic conduction converting elements. Nonevoked pulse bursts were observed with amplitudes in the 10 to 200 microvolts range. An upper limit value of the cell refractory period has been estimated from the maximum pulse frequency observed.The existence of evoked action potentials in plants due to a stimulation of the cell depolarization-repolarization process is well established. The concept of stimulation covers a broad range of interactions between the cell and its environment. In some cases the act of observation might be considered a stimulation in itself because of the interaction present during the observation. The general bioelectrical measurement situation is influenced by the not negligible interaction between the measurement apparatus and the object under investigation.In the present work, measurements have been made of nonevoked bioelectrical signals in the tissue of the India-rubber tree (Ficus elastica). Nonevoked in this case means that no stimulus was applied to the plant except for the presence of the electrodes in the petioles and the bias current of the amplifiers which is less than 10" amp. Bursts of pulses with well defined and rather constant amplitudes have been found to be produced by bioelectrical generators located in the plant tissue. Pulse burst lengths ranging from 30 sec to 25 min have been observed. The frequency of occurrence of the pulses within the burst is in the 0.5 to 200 pulses per minute range. Great effort was made to ensure that pulses generated by sources external to the plant were not mistaken for signals generated in the plant. Because of the electrode arrangement used, the pulses observed are assumed to represent the cellular action potential produced by groups of cells, analogous to the extracellular action potential as defined in medical electrophysiology (3). This assumption is substantiated by the fact that, while the single plant cell polarization-depolarization action potential has an amplitude of about 70 mv (6,14), the potentials measured in the course of the present work were all in the 10 to 200 /tv range. A difference in amplitude of the same order of magnitude between the intra-and extra-cellular action potentials is also found in medical electrophysiology (1 1).As in animal tissue, the electrical activity in plant tissue is based on ion transport mechanisms (8). Due to this circumstance, it is necessary to make a conversion from the ionic conduction present in the tissue to the electronic conduction which occurs in the measuring circuit, in order to measure electrical effects in the plant tissue. This conversion is accomplished at the tissue-electrode interface. The electrodes should preferably perform this conversion without disturbing the ionic concentrations or permanently damaging the plant tissue. In the present work, metal electrodes made of stai...