Ventilatory sensitivity to CO^sub 2^ in awake adult Brown Norway (BN) rats is 50-75% lower than in adult Sprague-Dawley (SD) and salt-sensitive Dahl S (SS) rats. The purpose of the present study was to test the hypothesis that this difference would be apparent during the development of CO^sub 2^ sensitivity. Four litters of each strain were divided into four groups such that rats were exposed to 7% inspired CO^sub 2^ for 5 min in a plethysmograph every third day from postnatal day (P) 0 to P21 and again on P29 and P30. From P0 to P14, CO^sub 2^ exposure increased pulmonary ventilation (VE) by 25-50% in the BN and SD strains and between 25 to over 200% in the SS strain. In all strains beginning around P15, the response to CO^sub 2^ increased progressively reaching a peak at P19-21 when VE during hypercapnia was 175-225% above eucapnia. There were minimal changes in CO^sub 2^ sensitivity between P21 and P30, and at both ages there were minimal between-strain differences. At P30, the response to CO^sub 2^ in the SS and SD strains was near the adult response, but the response in the BN rats was 100% greater at P30 than in adults. We conclude that 1) CO^sub 2^-sensing mechanisms, and/or mechanisms downstream from the chemoreceptors, change dramatically at the age in rats when other physiological systems are also maturing (~P15), and 2) there is a high degree of age-dependent plasticity in CO^sub 2^ sensitivity in rats, which differs between strains. PUBLICATION ABSTRACT