This study documents the effects of routine nursing procedures and loud noise events on the behavioral and physiological responses of premature infants in the neonatal intensive care unit (NICU). The subjects were 55 premature infants ranging in weight from 480 to 1930 g and in age from 23 to 37 weeks gestation. Nineteen nursing activities common in the NICU as well as loud noises such as alarms, telephones, loud speech, or infant crying were recorded every 5 minutes. The infants' physiological and behavioral responses were recorded at 5-minute intervals for 2 hours in the morning and 2 hours in the evening. Nursing interventions and noise resulted in significant changes in both the behavioral and physiological responses of infants. The presence of noise alone and nursing interventions alone resulted in similar physiological responses; however, the combination of these events was not cumulative. Infants changed their behavioral states an average of six times each hour during the 12 observation periods, and the number of enduring states (10 minutes or longer) averaged 10 times in the 48 observation periods of 4 hours.