This investigation was performed to determine whether hypercapnic exposure elicited expression of the c-fos protooncogene product, FOS, in nucleus of the solitary tract (NTS) and area postrema (AP) neurons of developing swine. Mean arterial blood pressure (MAP) and heart rate (HR) were also monitored to evaluate whether numbers of neurons containing FOS were related to changes of MAP and HR. In each experiment, two litter-matched piglets were prepared simultaneously, i.e., Saffan anesthesia, paralysis, and artificial ventilation (100% O(2)). One animal was exposed to hypercapnia (1 h of 10% CO(2), balance oxygen), while the other continued to breathe 100% O(2). Animals were studied at three different ages: 5-8 days, 13-15 days, and 26-34 days old. In the NTS, FOS expression was prominent in regions corresponding to the general visceral afferent subdivision; the AP showed no such topographic distribution. The number of NTS and AP neurons with FOS in hypercapnic-exposed animals was significantly greater than those of unexposed animals. However, an age-related increase of FOS was observed only for NTS neurons, with the greatest number observed in 13- to 15-day-old animals. Increases of MAP, not HR, were noted during the early part of hypercapnia in the 5- to 8-day-old group; older animals exhibited no change of MAP. Our findings demonstrated that prolonged hypercapnic stimulation elicited FOS expression in AP and NTS neurons of developing animals, and that such expression was non-uniform, depending upon the region studied.