1. The effects of tetrabutylammonium (TBA) on the mechanical tension and on the electrical behaviour of endothelial and smooth muscle cells were studied in intact porcine coronary artery strips.

2. Superfusion of strips with TBA (2‐20 mM) induced mechanical oscillations, leading to an increase in tonic isometric tension.

3. TBA‐induced mechanical oscillations were correlated with fluctuations of the membrane potential of endothelial cells, which were identified by iontophoretic injection of Lucifer Yellow.

4. The endothelial cell membrane potential fluctuations appeared as action potentials or smaller amplitude slow waves, and were synchronized with electrical membrane potential fluctuations of the underlying coronary smooth muscle cells.

5. Oscillations induced by TBA in smooth muscle cells were not affected by removal of the endothelium, and depended on the presence of calcium in the external medium.

6. To our knowledge, this is the first description of action potential‐like fluctuations in the endothelium. It is concluded that the oscillations were generated in the smooth muscle and that they propagate to the endothelium. The question of the mode of propagation of the signal is discussed.