Action potentials generated spontaneously (SAPs) and evoked by electrical stimulation (APs) in tomato plants (Solanum lycopersicum L.) cv. Micro-Tom ABA-deficient mutants (sitiens-MTsit) and its wild type (MTwt) were characterized by continuous monitoring of electrical activity for 66 h and by application of an electrical stimulation supplied extracellularly. MTsit generated SAPs which spread along the stem, including petioles and roots with an amplitude of 44.6 +/- 4.4 mV, half-time (t1/2) of 33.1 +/- 2.9 s and velocity of 5.4 +/- 1.0 cm min(-1). Amplitude and velocity were 43 and 108 % higher in MTsit than in MTwt, respectively. The largest number of SAPs was registered in the early morning in both genotypes. MTsit was less responsive to electrical stimuli. The excitation threshold and the refractory period were greater in MTsit than in MTwt. After current application, APs were generated in the MTwt with 21.2 +/- 2.4 mV amplitude and propagated with 5.6 +/- 0.5 cm min(-1) velocity. Lower intensity stimuli did not trigger APs in these plants. In MTsit APs were measured with amplitude of 26.8 +/- 4.8 mV and propagated with velocity of 8.5 +/- 0.1 cm min(-1).