Investigation of respiratory control requires an accurate measuring system which does not itself alter the pattern of respiration. We have used a transthoracic impedance pneumograph to investigate the effects of application of a face mask plus pneumotachograph or the face mask rim alone on tidal volume and respiratory frequency in 10 sleeping term infants aged 1-4 days. Application of the face mask rim led to a small but significant fall in respiratory frequency, 12% in quiet sleep (QS) and 17% in rapid eye movement (REM); with the application of the complete face mask plus pneumotachograph, changes were 10% and 14% in QS and REM, respectively. These changes were accompanied by rises in tidal volume of similar magnitude (15%, 11% in QS and 21%, 17% in REM, respectively). These results suggest that the change in respiratory pattern is due mainly to trigeminal stimulation rather than respiratory loading; thus, recordings obtained by methods utilising face masks cannot be considered to represent undisturbed respiration, for at least the first 15 min of recording.