THE wound response of several plant species involves the activation of proteinase inhibitor (pin) genes and the accumulation of pin proteins at the local site of injury and systemically throughout the unwounded aerial regions of the plant1,2. It has been suggested that a mobile chemical signal is the causal agent linking the local wound stimulus to the distant systemic response, and candidates such as oligosaccharides3, abscisic acid4 and a polypeptide5,6 have been put forward. But the speed of transmission is high for the transport of a chemical signal in the phloem. The wound response of tomato plants can be inhibited by salicylic acid7 and agents like fusicoccin that affect ion transport8, and wounding by heat9 or physical injury produces electrical activity that has similarities to the epithelial conduction system10 used to transmit a stimulus in the defence responses of some lower animals11. Here we design experiments to distinguish between a phloem-transmissible chemical signal and a physically propagated signal based on electrical activity. We show that translocation in the phloem of tomato seedlings can be completely inhibited without effect on the systemic accumulation of pin transcripts and pin activity, and without hindrance to propagated electrical signals.