Redistribution of air within the lungs may potentiate "fright" bradycardia in submerged crocodiles (Crocodylus porosus).

1. Voluntary undisturbed dives by Crocodylus porosus were short in duration (3.08 +/- 1.87 min, mean +/- SD) and accompanied by a small but significant bradycardia (14.3 +/- 5.9% drop). 2. When crocodiles were disturbed underwater there was a rapid onset of "fright" bradycardia, to 65 +/- 6.0% of surface heart rates and dive durations were prolonged to 19.6 +/- 1.8 min. 3. The development of "fright" bradycardia was not accompanied by any increase in intratracheal pressure or expulsion of lung gas. However, sustained contraction of the abdomen and expansion of the thorax revealed a redistribution of air anteriorly within the lungs. 4. We propose that the redistribution of air within the lungs may generate an afferent signal which potentiates the initiation of a severe, dive-prolonging bradycardia.