Changes of membrane properties induced by hypoxia and hypercapnia and/or acidosis were tested on central neurons and glial cells in-vivo and on neurons of hippocampal slices as well as on cultured isolated sensory spinal ganglion (SSG) cells in-vitro. With hypoxia neocortical and spinal neurons as well as glial cells in-vivo depolarized. The same reaction was found in CA3 neurons of hippocampal slices located in layers remote from the bath fluid. The membrane potential of SSG cells were highly insensitive to lowering of bath pO2. The developing response was related to changes of K+ concentration in the extracellular fluid. With hypercapnia the overwhelming majority of neocortical and spinal neurons in-vivo hyperpolarized with the postsynaptic potentials being reduced. In these preparations glial cells depolarized. The extracellular K+ concentration increased which may in part be due to an increase in neuronal K+ conductance. When the extracellular milieu of superficial CA3 neurons in hippocampal slices was predominantly determined by the ionic composition of the bath fluid, hypercapnia depolarized these cells whereas neurons in the innermost layers of the slice hyperpolarized. SSG cells depolarized when pH in the bath was lowered. When pCO2 was elevated at constant bicarbonate concentrations in the bath, SSG cells depolarized as well. In bath fluid, however, containing buffer proteins SSG cells were found to hyperpolarize during hypercapnia. Hyperpolarization occurred also when the bicarbonate concentration in the bath fluid was raised during hypercapnic periods.