The extracellular K+ concentration, ([K]e), in the brain cortex of rats at different ages was measured by means of K+-sensitive microelectrodes. [K]e was between 3 and 5 mM at all ages. Following nitrogen inhalation there was an increase in [K]e which exhibited a sigmoid pattern in every age group: Firstly, there was a slow rate of rise which was followed by a sudden, steep increase where the [K]e rose to about 70 mM. During the subsequent 5-10 min a plateau value of about 90 mM was reached. The prominent difference between the age groups was the time until the steep rise began. The time to the steep increase in [K]e and the [K]e at the start of the steep was inversely related to the age of the animal. A close relation was found between the time to the start of the steep increase and the time to the last gasp during nitrogen breathing (r = 0.98). It is suggested that the different resistence to anoxia in young and adult animals is related to differences in the ability to keep near normal potassium gradients across the cells in the brain.