Inspiratory activity on the left and right sides must be coordinated to be effective. We used cross-correlation to examine the hypothesis that the coordination of left and right medullary inspiratory neurones is produced by excitation from common sources and by midline-crossing excitatory connections among these neurones. In adult rats, a total of 185 contralateral pairs of inspiratory neurones (n=370) were recorded extracellularly, and classified, according to their firing pattern, as augmenting (n=262), constant (n=82) or decrementing (n=26). Of the 262 augmenting inspiratory neurones, 98 were classified as phrenic premotor neurones by cross-correlation with phrenic nerve discharge. The 185 cross-correlograms showed little evidence of common activation, or midline-crossing excitatory connections. Of the 45 cross-correlograms for pairs of augmenting neurones, only 4 (≅9%) indicated a common activation, and only one a monosynaptic connection. Of the 45 for pairs of augmenting and phrenic premotor neurones, only 9 (20%) showed a common activation, and only 2 a monosynaptic excitatory connection. Of the 19 pairs of phrenic premotor neurones, 5 from the same rat showed high-frequency oscillations, and 1 a monosynaptic excitatory connection. Cross-correlograms for pair combinations of other types of neurones also exhibited few features. We suggest that, in the adult rat, although both common activation and excitatory cross-connections exexist as a means for coordinating left and right ventral group inspiratory neurones to the same respiratory rhythm, they are insufficient to account for it.