Humming is probably more effective than speech improving mothers' cardiorespiratory function and infants' self-regulation. We intend to understand the effects of: 1) maternal humming vs. speech on preterm infants’ physiological parameters; 2) maternal humming vs. speech on mothers’ physiological parameters; 3) humming melodic contours and of the lengthening of the final note on preterm infants’ physiological parameters. This study was designed as a single group repeated measure, using microanalytical methodology (ELAN software), with a protocol (silent baseline / speech or humming / silence / humming or speech / silence) applied to preterm dyads (N = 36). Audio and video observations were recorded. Infants’ and mothers’ heart rate (HR) and O2 saturation were observed once a minute. Proportion of O2 saturation relative to HR (Prop. O2 saturation/HR) was estimated for both partners during protocol. We found an infants’ HR mean significantly lower during humming (p = .028), a significantly higher Prop. O2 saturation/HR during humming for infants (p = .027) and for mothers (p = .029). The duration of sinusoidal contours together with the lengthening of the final note predict infants’ Prop. O2 saturation/HR. Musical features of humming seem to improve the physiological stability of preterm infants during kangaroo care.