1. Changes in membrane and action potentials of cat spinal motoneurones during acute asphyxiation and re-oxygenation were recorded with an intracellular technique.2. The asphyxial potential of the grey matter, which develops in the first 2-2.5 min of asphyxiation, can be expected to interfere with the membrane potential record. After correcting for this effect a gradual depolarization of the soma at a rate of 3-4 mV/min was found, commencing within a fraction of a minute after the start of asphyxiation.3. The orthodromic responses of the motoneurones were the most vulnerable to O(2) lack. They failed earlier than the responses to antidromic and to direct excitation of the cell through the micro-electrode. After failure of the orthodromic spike an excitatory post-synaptic potential remained for a short time. Failure of antidromic excitation began by the dropping out of the some dendritic potential, followed by the arrest of the initial segment response.4. It was concluded that the early arrest of orthodromic excitation is caused by presynaptic failure.5. All changes in membrane and action potentials were completely reversible by re-oxygenation after periods of asphyxia lasting from 4 to 6 min. The orthodromic response recovered markedly slower than the antidromic and direct ones.