Apnea is a common complication in infants infected by respiratory syncytial virus (RSV). Intranasal inoculation of RSV in conscious weanling rats strengthens the apneic responses to right atrial injection of capsaicin (CAP), leading to 66% mortality. The objectives of the present study were to determine (1) whether RSV infection changes baseline minute ventilation (V_E) and arterial blood gases; (2) what the effects of RSV infection are on the dynamic respiratory responses to CAP; and (3) whether the RSV-strengthened apneic responses are age-dependent. Our experiments were conducted in anesthetized and spontaneously breathing rats divided into four groups of weanling and adult rats that received either intranasal inoculation of RSV or virus-free medium. Two days after RSV infection (0.7 ml kg^-1), animal blood gases, baseline V_E, and V_E responses to right atrial injection of three doses of CAP (4, 16, and 64 µg kg^-1) were measured and compared among the four groups. Our results showed that RSV infection increased respiratory frequency (~ 25%, P < 0.05) in weanling, but not adult rats, with little effect on arterial blood gases. RSV infection amplified the apneic responses to CAP in weanling, but not adult rats, characterized by increases in the initial (40%) and the longest apneic duration (650%), the number of apneic episodes (139%), and the total duration of apneas (60%). These amplifications led to 50% mortality (P < 0.05). We conclude that RSV infection increases respiratory frequency and strengthens the apneic responses to CAP only in anesthetized weanling, but not adult rats.