In vivo mammalian preparations can exhibit eupnea, apneusis and gasping. In vitro mammalian preparations exhibit only a single invariant pattern, which appears identical to gasping. We characterized the patterns of ventilatory activity of a perfused heart-brainstem preparation of the juvenile rat. In this preparation, phrenic activity has a 'ramp-like' rise similar to eupnea in vivo. Peak phrenic activity declines and ultimately disappears in hypocapnia. In hypercapnia, both frequency and peak of phrenic bursts increase. In hypoxia, such increases are transient. The phrenic burst is terminated by electrical stimulation of the pontile 'pneumotaxic center' and, as in apneusis, is prolonged by lesions in this region. With severe hypoxia or ischemia, the 'ramp-like' phrenic activity is replaced by the 'decrementing' pattern of gasping. Variables of phrenic activity in gasping produced in hypoxia and ischemia are identical. We conclude that the perfused juvenile rat preparation exhibits patterns of eupnea, apneusis and gasping which are similar to in vivo mammalian preparations.