In many neonatal mammals, including humans and rats, there is a developmental increase in the ventilatory response to elevated pCO2. This maturation of central respiratory chemoreception may result from maturation of intrinsic chemosensitivity of brainstem neurons. We have examined age-related changes in chemosensitivity of neurons from the rat medullary raphe, a putative site for central chemoreception, using perforated patch-clamp recordings in vitro. In brain slices from rats younger than 12 days old, firing rate increased in 3% of neurons and decreased in 17% of neurons in response to respiratory acidosis (n = 36). In contrast, in slices from rats 12 days and older, firing rate increased in 18% of neurons and decreased in 15% of neurons in response to the same stimulus (n = 40). A tissue culture preparation of medullary raphe neurons was used to examine changes in chemosensitivity with age from three to 74 days in vitro. In cultured neurons younger than 12 days in vitro, firing rate increased in 4% of neurons and decreased in 44% of neurons in response to respiratory acidosis (n = 54). In contrast, in neurons 12 days in vitro and older, firing rate increased in 30% of neurons and decreased in 24% of neurons in response to respiratory acidosis (n = 105). In both types of chemosensitive neuron ("stimulated" and "inhibited"), the magnitudes of the changes in firing rate were greater in older neurons than in young neurons. These results indicate that the incidence and the degree of chemosensitivity of medullary raphe neurons increase with age in brain slices and in culture. This age-related increase in cellular chemosensitivity may underlie the development of respiratory chemoreception in vivo. Delays in this maturation process may contribute to developmental abnormalities of breathing, such as sudden infant death syndrome.