Ventilatory sensitivity to CO(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(2) sensitivity. Four litters of each strain were divided into four groups such that rats were exposed to 7% inspired CO(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(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(2) increased progressively reaching a peak at P19-21 when Ve during hypercapnia was 175-225% above eucapnia. There were minimal changes in CO(2) sensitivity between P21 and P30, and at both ages there were minimal between-strain differences. At P30, the response to CO(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(2)-sensing mechanisms, and/or mechanisms downstream from the chemoreceptors, change dramatically at the age in rats when other physiological systems are also maturing ( approximately P15), and 2) there is a high degree of age-dependent plasticity in CO(2) sensitivity in rats, which differs between strains.