Apnea and respiratory failure often occur in infants with pulmonary disease. Bronchopulmonary C-fiber (PCF)-mediated apnea is an important component of respiratory dysfunction. This study was undertaken to define the postnatal development of PCF-mediated apnea. The experiments were conducted in five groups of anesthetized, tracheotomized, and spontaneously breathing rats with ages at postnatal days P1 - 3; P7 - 9, P14 - 16, P21 - 23, and P56 - 58, respectively. Right atrial bolus injection of three doses of capsaicin (CAP), equivalent to 2, 4, and 8 µg kg^-1 used previously in 450-g rats, was applied to stimulate PCFs. We found that: (a) CAP-induced apneic response (T^E) and the sensitivity of this response (T_E% µg^-1) were significantly greater in the rats < P10 than those > P10; (b) the CAP-induced apneas were vagally dependent in all rats tested; and (c) bivagotomy-induced prolongation of T^E was much greater in the rats < P10 than those > P10. From these findings we concluded that as compared to the older rats (> P10), the newborn rats have a stronger PCF-mediated respiratory inhibition that may contribute to infants vulnerability to respiratory failure.