This exploratory longitudinal study was designed to compare the neonatal illness severity, sleep-wake, and respiratory sleep behaviors of preterm infants whose mothers received prenatal corticosteroids and/or magnesium sulfate (MgSO4) with those of infants whose mothers did not receive these medications. The 134 infants were divided into four groups: those whose mothers received MgSO4 only, those who received steroids only, those who received both MgSO4 and steroids, and those who received neither. The groups did not differ on infant characteristics or illness severity. Infants exposed to MgSO4 had more active sleep without rapid eye movement, indicating poorly organized active sleep. The MgSO4 -only group had higher quiet sleep regularity scores and fewer state changes. These findings suggest that fetal exposure to MgSO4 may subtly affect the central nervous system.