In the past 5 years there has been an enormous increase of evidence that the ion channels activated by mechanical force are common to a wide variety of cell types. Mechanosensitive (MS) ion channels form a small proportion of the total channel population. They are now found in more than 30 cell types from E. coli, yeast, to plant, invertebrate, and vertebrate cells, where they occur in virtually all types of cells from bone to smooth muscle, as well as neurons. The majority of MS channels are permeable to monovalent cations and are slightly selective for K+ over Na +. How­ 2 ever, there are several reports of anion-selective MS channels, MS Ca + channels, and MS channels with large conductances that do not dis­ criminate markedly between cations and anions. Recently B. Hille has postulated possible evolutionary relationships between several types of ion channels, with mechanosensitive channels predating even the eukaryotes. Two voltage-gated channel types originate with the stem eukaryotes, as deduced from the presence of voltage-gated K+ 2 and Ca + channels in protozoa, algae, or higher plants. Agonist-gated chan­ nels as well as voltage-gated Na + channels appear with the earliest metazoan animals, as deduced from the presence of Na + spikes and fast chemical synapses in cnidaria (coelenterates), ctenophores, and all higher animals.