African lungfish, Protopterus dolloi exhibited constant rates of O 2 consumption before (0.95±0.07mmolkg −1 h −1 ), during (1.21±0.32mmolkg −1 h −1 ) and after (1.14±0.14mmolkg −1 h −1 ) extended periods (1–2 months) of terrestrialization while cocooned. Although a breathing event in terrestrialized fish consisted of multiple bouts of inspiration and expiration in rapid succession, the mean frequency of pulmonary breathing events was unaltered in the terrestrialized fish (16.7±1.4h −1 versus 20.1±4.9h −1 in the aquatic and terrestrialized fish, respectively). Hypoxia (∼20mmHg) increased the frequency of breathing events by 16 and 23h −1 in the aquatic and terrestrialized fish, respectively. Hyperoxia (∼550mmHg) decreased breathing event frequency by 10 and 15h −1 in the aquatic and terrestrialized animals. Aquatic hypercapnia (∼37.5mmHg) increased pulmonary breathing frequency (from 15.3±2.3 to 28.7±5.4h −1 ) in free swimming lungfish, whereas aerial hypercapnia was without effect in aquatic or terrestrialized fish.