Measurement of breathing volumes in neonatal mice is of growing importance in order to characterize the influence of development and genetic modifications upon respiratory control in order to evaluate hypotheses concerned with human infant deficits that may affect Sudden Infant Death Syndrome (SIDS) for example. Current techniques require undesirable physical constraints or incur possible artifacts specific to very small animals. We have examined the utility of a recently proposed approach using an acoustic resonance procedure that does not require undue physical constraint beyond placement in the acoustic plethysmograph. We show here that this approach can be applied to baby mice five days after birth and that it can be accurately calibrated. In addition, this approach should be useful to study unrestrained neonatal mice under conditions where body temperature approaches environmental temperature and barometric plethysmography cannot be used.