In vivo, the augmenting pattern of integrated phrenic nerve discharge of eupnea is altered to the decrementing pattern of gasping in severe hypoxia or ischaemia. Identical alterations in phrenic discharge are found in perfused in situ preparations of the juvenile rat. In this preparation, gasping was produced by equilibration of the perfusate with various levels of carbon dioxide and oxygen. The duration of the phrenic burst, the interval between bursts and the burst amplitude were not significantly different following equilibration with 21-6%O(2) at 5% CO(2) or with 0-9% CO(2) at 6% O(2), with the exception that the burst amplitude was significantly greater in hypercapnic-hypoxia (9% CO(2) at 6% O(2)). It is proposed that hypoxia-induced gasping results from the release of an endogenous pacemaker activity of rostral medullary neurons. This release is caused by cellular mechanisms that change the balance between membrane ionic currents. Moreover, these cellular mechanisms may be explicitly induced by alterations in the ionic and metabolic homeostasis.