The observed low frequencies of action potentials observed in medullary respiratory neurons of immature opossums (Didelphis virginiana) could occur because these cells are incapable of achieving higher sustained firing rates. Nonsustainability of firing might also help explain why the inspired breath is brief (approximately 0.1 s) in the youngest opossums and rises very slowly during postnatal life. Firing frequencies of medullary respiratory neurons were examined in spontaneously breathing thiobarbiturate-anesthetized opossums before and after stimulation by the glutamate agonists, N-methyl-D-aspartate (NMDA; 20 mM) or kainic acid (KA; 0.5 mM). Drugs were applied using progressively larger pressure injections through a micropipette; animals were tested from the 5th postnatal wk to adulthood. With a sufficient injection volume, stimulation of cell firing would be followed by apparent suppression of action potentials. A maximum "sustained" firing frequency was obtained from the last injection where discharge remained elevated for at least 0.5 s. Inspiratory and expiratory neurons tested with either drug showed the lowest rates of firing in opossums at 5-9 wk of age compared with 10- to 14-wk-old animals and/or adults. Despite higher rates of discharge in 10- to 14-wk-old animals and/or adults, maximum sustained neuronal firing in the youngest animals was at a higher frequency than during spontaneous breathing and, at least in the cell population tested, does not represent a limitation that might affect breathing pattern.