Purpose: The study was done to measure the effects of maternal heart sound on body weight, physiologic reactions (heart rate [HR] and cortisol) and behavioral states of preterm infants. Methods: Thirty-five preterm infants were recruited from a neonatal intensive care unit at a university hospital. Institutional Review Board approval and informed consent were obtained. The infants were assigned to an experimental group (n=18) with an auditory stimulation for 7 days of life, a continuous delivery of maternal heart sound using MP3 attached inside the incubator, or to a control (n=17) without any auditory stimulation. The outcome variables, daily variations in weight, HR and behavioral states, and differences in cortisol were analyzed. Results: There were differences in variations of daily weights (F=3.431, p=.011) and in cortisol (t=3.184, p=.006) between groups, but no difference in variations of daily HR (F=0.331, p=.933) and behavioral states (F=1.842, p=.323). Conclusion: The findings support the safety of continuous maternal heart sound as no changes in HR and behavioral states occurred, and the efficacy as weight increased and cortisol decreased. This auditory simulation may lead to more efficient utilization of energy in preterm infants by consistently providing familiar sounds from intrauterine life and blocking noxious sounds from NICU environments.