In conscious animals, respiratory frequency (f) and tidal volume (VT) vary breath to breath. Examining the average value of variables associated with specific bins of another variable, such as breath f, provides a unique tool to examine respiratory behaviour. In conscious Sprague-Dawley rats respiratory breath timing, tidal volume (VT) and drive (VT/TI) were characterized using a plethysmograph. In the majority of rats at low breath f, expiratory time (TE) exceeded inspiratory time (TI) and these times became equal as f exceeded 150 breaths/min; there was no evidence for TI greater than TE at higher f, as observed in cats and dogs. When VT is normalized per kg, rat breath VT and VT/TI, binned by breath f, are continuous with those for the cat and non-panting dog at the lowest breath f. Relative to breath f, breath VT and VT/TI in rats are greater than in normothermic panting dogs (20 degrees C), but only slightly greater than those variables in panting dogs in the heat (30 degrees C). Lower values of breath VT/TI, binned by breath f or V, in cats and dogs are compensated for by a greater TI relative to the duration of a given breath. This comparative analysis suggests continuities of respiratory pattern generation among species.