The aim of this study was to determine the analgesic and physiologic effects of warmth in neonates undergoing intramuscular injection of vitamin K. Appropriate for gestational age infants of 37/1- 42 weeks of gestational age were included. Infants of mothers with substance abuse, infants of diabetic mothers, neonates with asphyxia, first and fifth minute Apgar score less than 7 were excluded. We observed the behavioral state only after the infant had spontaneously reached 1 of 3 quiet behavioral states (State 1: eyes closed, regular respiration, no movements; state 2: eyes closed, irregular respiration, small movements; or state 3: eyes open, no movements). Forty infants were divided in control and intervention groups in the first hour after birth. The protocol consisted of baseline (2 min), intervention (2 min), injection (20 s ), and recovery (2 min) periods. Infants in the intervention group were warmed by radiant warmer, first in manual mode with 100% power for 2 minutes followed by servo controlled mode with 35 degrees. Infants in the control group were also warmed by radiant warmer but only at 35 degrees in servo controlled mode during the study. Maximum heart rate and oxygen saturation were recorded two minutes before, during and two minutes after the injection in thirty-second intervals . Pain measurement were recorded independently (with NIPS scale) through video recording.