Barium has long been used to study calcium influx, since it passes through calcium channels more readily than calcium itself (Eckert & Lux, 1976). It also inhibits potassium channels, making it easier to isolate the inward current. Barium entry into non-excitable cells has been studied using Fura-2 over the last decade (e.g. Bakowski & Parekh, 2007) yet little seems to be known about intracellular Ba²⁺ ions in nerve cells. I will show that Ca²⁺-sensitive microelectrodes (CaSM) made with the sensor ETH 129 (Ammann et al, 1987) appear to be almost equally sensitive to Ba²⁺ as to Ca²⁺, as shown in the illustrative result in Fig 1. This experiment was done on a snail neurone in an isolated ganglion preparation using techniques as described in Thomas (2002) and Thomas & Postma (2007). Strangely, CaSMs made with this sensor were originally described as very much less sensitive to Ba²⁺ than to Ca²⁺ ions. In the experiment illustrated a large neurone was penetrated first with a CaSM, then with two conventional CsCl-filled microelectrodes to measure the membrane potential and clamp it at -50 mV, and finally with a pH-sensitive microelectrode (pHSM). Every few minutes the cell was depolarised to zero for 5 s. This opened calcium and potassium channels. Initially the cell was bathed in normal 7 mM Ca snail Ringer and depolarised several times, only the last of which is shown. Then the superfusate was changed to a Ringer in which all the calcium was replaced by barium. The CaSM potential changed little, but the depolarisation-induced elevations in CaSM potential were larger than seen in calcium, and were accompanied by a larger change in pH_i. The recovery of the CaSM signal after each presumed barium influx was slower that after each calcium influx. The clamp current during the depolarisations was outward in calcium, but inward in barium. The effects of barium Ringer were reversed on return to normal calcium Ringer. The CaSMs were made from quartz micropipettes silanized at about 450°C for 10 min, while the pH-sensitive microelectrodes were made from borosilicate glass micropipettes silanized at about 250°C. The quartz micropipettes were backfilled with 1 mM CaCl₂, and then frontfilled by suction with a 0.1-0.3 mm column of cocktail (Thomas 2002) and left for at least an hour before use. The pHSMs are made similarly with the cocktail Fluka 95297.