Mothers around the world sing to infants, presumably to regulate their mood and arousal. Lullabies and playsongs differ stylistically and have distinctive goals. Mothers sing lullabies to soothe and calm infants and playsongs to engage and excite infants. In this study, mothers repeatedly sang Twinkle, Twinkle, Little Star to their infants (n = 30 dyads), alternating between soothing and playful renditions. Infant attention and mother–infant arousal (i.e., skin conductivity) were recorded continuously. During soothing renditions, mother and infant arousal decreased below initial levels as the singing progressed. During playful renditions, maternal and infant arousal remained stable. Moreover, infants exhibited greater attention to mother during playful renditions than during soothing renditions. Mothers' playful renditions were faster, higher in pitch, louder, and characterized by greater pulse clarity than their soothing renditions. Mothers also produced more energetic rhythmic movements during their playful renditions. These findings highlight the contrastive nature and consequences of lullabies and playsongs.