1 The effect of altering the ionic balance of the cerebrospinal fluid (CSF) on cloacal temperature of unanesthetized pigeons kept at room temperature (20-25 degrees C) was examined by injection or infusion of solutions of different ionic composition into a cannulated lateral cerebral ventricle. 2 An increase in the concentration of calcium ions caused a fall in temperature and behavioural sedation. The effects were the same whether the calcium was present as calcium chloride or as the calcium disodium salt of ethylenediamine tetra-acetic acid (CaNa2EDTA). 3 When the concentration of sodium ions in the CSF perfusate was increased by addition of NaCl or that of calcium ions was decreased by addition of Na2EDTA a rise in temperature was often produced but this was not consistent. NaCl sometimes had either no effect or lowered the temperature. Na2EDTA while producing a rise when first injected failed to do so when repeated a few hours, 24 h and often 72 h later. Prolonged infusion of either agent caused intense behavioural excitement leading to death. 4 Potassium ions, like sodium ions, caused a rise in temperature but only when infused continuously. Behavioural excitement was only rarely observed. 5 Magnesium produced a fall in temperature. The concentration required was much higher than that of calcium but the hypothermia was more prolonged suggesting a slower elimination of the magnesium ions from the CSF. Magnesium ions caused tremors, nystagmus and ataxia as opposed to sedation caused by calcium. 6 All these were central effects as they were not obtained when the substances were injected intravenously. 7 Since changes in body temperature of the pigeon produced by injection of calcium or sodium ions into the CSF were similar to those seen in various species of mammal, it is concluded that the relative concentration of these ions within the brain plays an important role in establishing the temperature setpoint in both birds and mammals.